Kasparov versus Karpov

In the following game, Kasparov fights Kasparov and went on with Rd3.                                            My initial thoughts: Why would white want to do this? According to my understanding, bishop and knight are “somewhat preferable” over 2 knights. Yes, I know that this position anyway favours bishop over knight and I see that the white knight is currently lacking a good square and once the white knight re-positions, it might get swapped-off the board anyway. So yes, the exchange in general favours white and might be inevitable.  But why now?  Black´s queen and rook are (mis-)placed behind the rook on c5 – so why free the line for them?

Regardless of all the above, I would judge the position being absolutely equal, so does the machine. Just for the sake of it, I played h4 - still things are absolutely equal (+0.15 for white).

So why does Nd3 receive an exclamation mark?