First off, it buys space. Sure, it's only a hex or two wide, but if you take Chinesau, that's a hex that will provide a ZoC, can't be knocked out of supply, has to be assaulted, and covers Odessa. It'll buy you an impulse for that big rush when you badly need time.
Finnish border lands: First off, there's always the chance that you'll get to conquer Finland. THat secures your northern flank, makes Leningrad almost unassailable.
And even if you can't take Helsinki, you'll almost certainly push the front line away from Leningrad, and you can cut off that Petsamo resource fairly easily.
And as others have said, each claim you make means one less resource for Germany and one more resource for the USSR. If you claim one of them on the first turn, between S/O 39 and M/J 41, you'll have "created" (actually a bit less due to rounding issues) 4.5 Soviet BP, and cost the Germans 8 BP. That's fairly significant. I usually do both border options as soon as I feel strong enough to beat up the respective countries I'm bullying. As a practical matter, that's usually S/O 39 for Bessarabia, and M/J or J/A 40 for Finland.