Tim Drake is the third character to have borne the name of Robin and a character so popular that he got an ongoing series 2 years before Nightwing, the original Robin.

After the brutal murder of the second Robin, Jason Todd, by the Joker in "Death in the Family", Bruce went into the usual obsessive loner routine. Realising that something must have happened to Robin, the 12 year old Tim Drake tracked down Dick Grayson and told him that Batman needed Robin.

Tim: "And when I see that without Robin Batman is going off the deep end, I know there's serious trouble. "
Dick: "Supposing all this is true, what do you want from me?"
Tim: "I think he needs you Dick. But he doesn't need Nightwing. He needs you as Robin." Batman #441, A Lonely Place of Dying Part 3

Tim: "BATMAN NEEDS ROBIN! Doesn't anyone understand?"
Alfred: "Perhaps, young man, perhaps Master Dick understands profoundly...perhaps that is why he brought you here." Batman #441, A Lonely Place of Dying Part 3

Dick having made a life for himself with the Titans, see "A Lonely Place of Dying" a Batman/Titans crossover, had no intention of returning to being Robin. This put Tim in the position where if there was to be a Robin, he was the only choice.

Bruce on the other hand had no intention of putting another boy in the firing lane, but Tim convinced him that he couldn't give criminals the impression that Robin could be killed.

Dick: "His instincts for detective work are astounding. "
Alfred: "And his acrobatic ability is quite remarkable. " Batman #442, A Lonely Place of Dying Part 5

In case you were wondering how Tim knew all about Batman & Robin. Tim was at the circus on the night Dick's parents were murdered and he had seen Dick do his special trick, a quadruple somersault, which only three people in the world could do. Later when he saw Robin do the same move he realised that if Dick is Robin then Batman must be Bruce Wayne.

Tim Drake has proved to be an extremely popular addition to the Batman family. The fact that for the first time Robin had a remotely sensible costume may have had something to do with his initial success, but many more have been won over by his personality.

It didn't take long for this new Robin to to receive a mini-series of his own, which took him off to Paris to learn the skills that he would need if he was going to survive as Robin. The first mini was written by Chuck Dixon who has written the character ever since and has, as yet. given no sign of being burnt out or tired of the character. The second mini even put him up against the man who killed his predecessor in the comics, although arguably it was the audience that killed Jason Todd.

The motivation in the comics for Robin getting a solo series was Azrael becoming The Batman: he didn't particularly want Robin around, and Tim didn't really want to be around this new, more violent Batman.

Robin: "This is the only downside to being Robin. I have to lie to my Dad. I hate it."
Nightwing: "Well, look at the bright side...you have someone to lie to." Robin #8, Knightsend Part 5

Tim Drake is an unusual member of the Bat Family because his father is still alive, which opens up possibilities for very different stories to be told. Normally when a new hero is created their family has to be killed off. It's easy to see why writers would want to kill of a potential hero's family: what greater motivation is there than the desire for justice, revenge, etc.; in our universe wearing a costume and fighting crime is not the action of a rational person; having a family makes it a lot harder to be a costumed crimefighter, if you are married you're spouse is likely to notice if you go out fighting crime every night and if their identity gets out your family will become the target of every person with a grievance.

Tim Drake has worked hard to keep his identity hidden: both his Robin identity hidden from his father and friends and conversely his Tim Drake identity, which he has managed to keep secret from Spoiler who knows him only as Robin. He has even managed to keep it secret from the information expert Oracle.

Tim is also following the tradition started by Nightwing in becoming the leader of his own team. Young Justice. YJ shows a much lighter side to the current Robin largely because of the other main members of the group; Impulse and Superboy neither of whom would ever be called serious. Robin usually has his work cut out trying to keep those two out of trouble.

Tim is the natural successor as Batman, in the unlikely event that Bruce ever retires. Dick is too much his own man and his relationship to the other Titans is too strong: they consider themselves a family. Tim is a natural detective, he found out all about Batman and Dick Grayson on his own, who has got where he is through hard work and determination..