Blood summon is not as essential an event as Purge, but it is still definitely useful. Its main role is to cover for some of the deck's weaknesses.

- It gives you a summoning spot on the front lines which Fallen Kingdom really needs.
- It gives you sudden reach.
- It is a sacrificial outlet.

It can also be very economically efficient with the right board and hand.  If you sacrifice an Undead Carrier to bring 2 Undead Warriors for example, you are getting +1 magic worth of value.
Is that worth the effort? Probably not, but it can be if it gives you other benefits. For starters those Undead Warriors would immediately get +1 boost for the destroyed Undead Carrier.

So it’s important to understand that while Blood Summon is mostly used for reach and as a sacrificial outlet, it can really be impressive if you also manage to get some economy out of it at the same time. However, don't put too much effort in trying to set up a 3+ common Blood Summon dream in most scenarios.