For 360 Manpower:
You get a unit that can easily kill early Flame-tracks and scout cars, which not only kills the damn vehicles but it saves you fuel, time, and manpower since you don't have to spend fuel and mp on researching At nades. As well you don't have to waste 2-3 at nades killing these vehicles, which roughly can cost around 50-75 munitions, which means more ppsh for your cons or dp28s.
Also, if they do manage to kill these vehicles, then they will vet up and are capable to support your vehicles and scale well during late game. Hell I saw Cataclaw, joking around and killing tigers with guards. Yes, not 1 but few tigers with a few vetted guards and 1 Atgun at his base. If he is reading this he probably remembers that dream team blob.
If you manage to have 3 guards with the isu and mark target ready. It can kill any vehicle easily even the tiger ace. Just mark it, button it, Isu shoots it, and repeat it with other guards. Yes it costs a lot of munitions but you tend to have a lot of munitions as soviets.
They are the reason why people decide to use guard rifle instead of (I forgot what it's called but it has shock, 120mm and ppsh)
If there is one thing I can change about guards, it would be that they don't drop their damn weapons every single time. It's ridiculous, that needs to be fixed other then that they are a great unit at the moment.
"You give them 1 vet they're good, if you give them 2 veterancy, they're great, and if you give them 3 veterancy, they're unbeatable."