"Certains" implies that there are "other ones". "Quelques" does not bring that precision".
For example, you have 10 tourists visiting a town, if 5 of them have lunch in a restaurant, you can say "certains sont allés au restaurant".
If you just say "quelques personnes seulement ont visité la cathédrale", it doesn't give any information about who they are, if they are a part or a group or not.
But the difference is quite subtle anyway !