Increasing bandwidth to vMotion operations decreases the duration of a vMotion operation. By decreasing the duration time, DRS is able to load balance more quickly.The time of olacing a host in maintenance mode will be reduced as well. DRS takes the vMotion time of a VM into account when calculating load balancing moves. It understands how quick it can migrate and thus allow more migration to happen in a single invocation. This will help DRS to get to a load-balanced state much quicker.
Recently I've written a couple of articles about vMotion, those might be interesting for you: http://frankdenneman.nl/vmotion/