I heard that using Tor is also a contribution because users will produce different activities to prevent being recognized by activity. Is it true?
The more people that use Tor, the bigger the crowd in which people can hide.
This also extends to activities. The more people there are doing the same thing - e.g. visiting the same websites - then, again, the less likely it is that one of those people will look unique.
For this to work well, everyone has to look the same to any external observer, which is where your browser fingerprint comes into play.