Why does LastPass use exactly 100100 PBKDF2 rounds
LastPass uses PBKDF2 with SHA256. The number of iterations (also called count) in PBKDF2(PRF=SHA256, Password, Salt, iterations, dkLen) is used against the password crackers. With the increase of the computation power, the developers change the number of iterations from 5000 to 100100 and there is nothing special about it other than being large. Having different numbers from others only can be considered a pre-countermeasure that may be helpful in the future. You can still adjust the iteration number to any value that you want.

The salt, on the other hand, is used as a countermeasure against Rainbow Tables. Actually the salt simply killed the Rainbow Tables. They are no longer helpful if the sites use a salt-based password hashing. 

With a good PBKDF function with properly used salt, the only way to break is the brute-forcing the possible password and combinations. The number of iterations linearly increases the cost of brute-force. That is, if you double, the attacker with the same system need double time.

Keep in mind that there is an old saying inside the US National Security Agency (NSA):

$$\texttt{Attacks always get better; they never get worse}$$