You can use it to spot prime. For example, shellac is the only thing to stop a knot bleed. But you can't use it on large surfaces outdoors. Some of the oil-based or alkyds are very inflexible also. It's also more effective than the other two primer-sealers on masonry, stucco, brick and aluminum siding.
Acrylics are also an effective primer for exterior concrete block.
However, concrete block can be a particularly tricky creature to tame. In Florida, they use a lot of sand, and you end up with a fairly tight block, not a lot of pores, and it's fairly easy to seal. But in Tennessee, where they don't have a lot of sand, they use a higher proportion of aggregate. You end up with a block that has good strength, but has so many pores. Sealing it is like glueing marbles together.