if a girl doesnt swallow she will be anxious to get off your dick and spit. Plus she isnt continuing the blowjob if shes collecting cum in her mouth so sensations at end lessen as she is waiting for it to all collect and not focused on blowjob anymore.
the worst part for a girl is not the blowjob but the cum. So if she just swallows she can continue the blowjob for a bit longer which makes a big difference in the feeling which is my experience anyways so prefer a girl who swallows