They're usually used as bulk capacitance, particularly at the input side of switching regulators. Thing is, their ESR is too high to be used alone so they're usually used in parallel with an MLCC ceramic. I've had success replacing both on the input stage with an aluminum polymer electrolytic (not standard aluminum) due to their low ESR and high capacitance (and better inrush rating). I usually use ceramics on the output.

[1] http://www.ti.com/lit/an/slta055/slta055.pdf

This is a great writeup on the various types from Kemet:

[2] http://www.kemet.com/Lists/TechnicalArticles/Attachments/5/A...