Do Candidates Have to Follow Party Platform?

I think the Platform is basically a sales pitch to get elected.

Some candidates take their favorite points of the platform and set them as a beacon which they actually try to reach.

But once someone is in office, I think it's a little like being in a canoe on a stormy ocean; you point to a place on the shore and say "I'm gonna land there". Then you paddle like hell while titanic forces batter you from every side, the canoe starts to leak and sharks circle. About half way through your struggle another election happens and you have to stop rowing for a while. At that point you are either disqualified from the race or allowed to struggle some more. Meanwhile you have drifted back out to your starting point and the goal has usually changed a bit so you have to start again. Nobody ever reaches the shore, all we do is edge toward a particular goal slowly, sometimes achieving small successes along the way.

Our job as voters is to pick the strongest rower who agrees with the goal we have in mind. But often times, no single candidate embodies those two qualifications.

Over the last few decades the candidates have been well known prior to the conventions taking place and now they have significant influence on what is or isn't in the platforms.

Do they have to follow them? There isn't any rule that says they do but they'd be in a tough spot if they stray very far from it. (The press would jump on it and you can bet the public would be aware of it in a hurry.)

Once the elections are over they are pretty much free to do whatever they want though. While both parties have platforms they don't really mean much beyond election day and are usually quickly forgotten. The press doesn't pay much attention to the platforms once the election is over with and, IMO, the general public is cynical enough that not many people actually believe that the platform is much more than a campaign gimmick.