It makes a hell of a whoopee cushion!
But seriously (as if talking about the gas that propels flatulence can ever be taken that seriously!) ...
Biogas can be used straight up for COOKING, LIGHTING GAS LAMPS, and running gas REFRIGERATION (like the Dometic that Culhane has at home in Germany). With a simple modifaction you can turn most gasoline engines into "trifuel engines" and run your genset on gasoline, propane, natural gas or biogas. You can use biogas with on demand gas heaters for showers or baths or dishwashing hot water. You could run your car, bus or truck or tractor off it (though you would have to find a way to compress and store it, like they do in Sweden).
Essentially you can use biogas for exactly the same things that people use natural gas for.
And why? Because biogas IS natural gas. It is in fact the only true NATURAL GAS -- the other being fossil fuel ('natural' in the prosaic sense, but hardly environmentally friendly in terms of extraction and net CO2 contribution).
There used to be natural gas air conditioners sold in the 1950s until the price of NG got too high. Now that we have biogas and also because the price of NG has gone down again, yes, you could run air conditioners on biogas.
So there -- there is very little we can't do with biogas. In some places they not only run taxis and buses off the stuff (like in the aforementioned Sweden) but run thousands and thousands of homes and businesses, creating both electricity and hot water using co-generation (this occurs in both the US and Germany among many other places!).
So what are you waiting for? Once you start producing your own biogas it is free energy, made from garbage. Go for it!
What can't you use biogas for, really?