I want a new Sonic game in the style of Adventure (the speed stages anyway), but I don't trust that Sonic Team currently has the ability to make a really good original game in that vein. An Adventure remake would be a good way for them to get reacquainted with making these types of games, so when they would make a new Adventure-like game, they could hone their skills and build on what they already learnt when making the remake.
With modern development tools, it would also be really fucking hard for them to screw up the remake and make a version that's worse than the original. Worst case scenario, we'd still at least get a better version of a classic game, even if the remake still wasn't what some might hope it was.