Circle Line works both ways.
It allows some people to bypass the two main lines. I, for one,
used to take the MRT from Buona Vista to the east in order to go
home from NUS. Now I take the Circle Line all the way to Paya Lebar
and bypass the city center in the process.
On the other hand, it also directs more people towards
the main lines. After all, the Circle Line is a 'feeder' line of
sorts for the main lines.
Let's say someone wants to go from Buona Vista to Ang Mo Kio. In
the past, he would go down to the city, change at Raffles
Place/City Hall, and take the northbound train. Now he would take
the CCL to Bishan and change trains there. So you could say that
the crowds have shifted from Raffles Place/City Hall to Bishan.
End result? I ain't gonna complain about the utility of the
Circle Line. I only wish they had actually built larger stations
and used six-car trains instead of three.