Indispensable if you find yourself often in the situation where you'd like to interact on websites you don't frequent but require an account to do so, and prefer not to keep enlarging the pile of sites you already have memberships on. Or, even more importantly, if you are unsure of the security of certain websites and do not want to endager your real e-mail account turning it into a spam-syphon or worse.
Super practical in those cases and very easy to use. So far (after several months of use) it has yet to fail me.
:)