I have never used the liquid, and haven’t used the old-fashioned kind in years, either, but since it’s mostly Phosphorus (I think some/all formulas probably contain Calcium, too) it can be thought of as a “root-and-bloom” formula, roughly speaking.
I have read that root crops (vegetables like carrots, potatoes…) benefit from it but have no first-hand experience; flowering plants should, too. Those that bloom and then fruit (tomatoes, e.g.) would be especially happy for the extra oomph. Transplants (those therefore needing to root in well) should also enjoy it.
All this must be disclaimed by the fact that it also depends on the soil the plants are growing in, and what nutrients are available and missing from it, and so on — so adding a Phosphorus booster isn’t the whole solution to healthy, productive plants if other things are missing/off.