Blockade

In theory, it should block any resources from that planet from reaching the blockaded empire - although I don't see any blockaded label as there was in SE4. It can also make the blockaded population unhappy as well.

Space Empires Depot | SE:V Balance Mod

It can repair and build at the same time.

Space Empires Depot | SE:V Balance Mod

Yes, the repair ability is a function of the spaceyard facility. It will automatically repair a certain tonnage in ships/facilities/units per turn depending on the level of your spaceyard. If you happened to destroy the spaceyard facility however ...

Why blockade the only planet in an empire?

If theres only the Homeworld, blockading is not effective granted, but say they had their home system plus 1 other and you blockaded that whole secondary system - you just denied them all kinds of resources/research and, if they were trying to support a large counter-offensive force, may just have tipped the scales where they are paying more in maintenance on those ships plus other facilites than they are making. Seems like a small thing but think about this: if you don't have enough resources to pay maintenance on a ship it can disband.

I've never had that happen because I've never been in a resource hole before but I read on these forums about how it could happen. Personally, I never blockade, if I'm attacking I'm glassing, lol. I find invasions to just be a hassle.

Myself, I like invading other empires. Never hurts to un-dome all those nasty toxic planets that seem to be everywhere. In SE IV, I'd run around with enormous invasion fleets and knock planets out all over the place.

SEV is different, however, mostly because the ground combat phase is a pain in the posterior. Especially dealing with homeworlds. Using Balance Mod, I generally pound the planet until all ground forces (troops and weapon platforms) are destroyed before I move in. But I need to drop so many troops to deal with the militia that my computer all but comes to a halt processing the battle.

Next game I may try using some plague bombs to soften up the populace for a few turns first... we'll see.

Invest in some Planetary Bombardment tech - Napalm is your friend

It depends on the population. I think it was 1 unit per 50 million people, or something in that direction...
So on huge worlds their might ba maaaany...

Ok. It sounds like you're describing a picket force to keep a lid on until you're ready to deal with it, not a blockade which is more economical in nature.

I found heavy weapons to be a more efficient use of ship space since it can also by used against ships and other targets. When you do a massive bombardment glass is still glass however you do it.

I mostly use just normal weapons too but I make certain ships have some planetary bombardment weps and the glassing of planets (particularly large/huge planets) go much quicker. The sheer amount of damage they put out can't be equaled by normal weps + if you really want to invade these babies can kill all the pop and just the pop, leaving the infrastructure. Course, I never invade so thats little use to me, heh.