What can someone do with router access?

I live in a shared house, there were problems with wireless running at slow speeds, the guys who lives in the house didnt know why, and were selfish and connected to the router directly with their ethernet cables, so i was left with no internet. Tried accessing 192.168.0.1, and it appeared that they had changed the admin password.

So i reset the router by pushing in the button in the back. I was then able to login to the router settings, i found out that the frequency channel was on 6, and not 11, so everything now works fine.

However a day after, i tried accessing 192.168.01, and theyve changed the address!! So i cant even access the router login page!!

Why would they do this???

What can they gain by having access to my mac abd ip address on these settings?.. Thats if my ip is on those settings..

Can they see what websites im visiting etc?