Unlike a hired, borrowed or service industry "rescue" wheelchair, it will be correctly configured and a known quantity which is always available and no panic at that railway station at 2am.

There can be other benefits too. Your spare powerchair can be "different" to your daily powerchair. For e.g. if your normal powerchair is a relatively slow, small powerchair intended mainly for "indoor" such as the majority of powerchairs are, then maybe your cheap old used spare one can be a bigger "off road" capable, full speed/range one designed primarily outdoor use. That way you have some choice in which chair you want to use depending on what your plans are. Flexibility is always a good thing.

Or for some (myself included) it makes more sense to use two (or three in my case) powerchairs that are the "same" or extremely similar for parts/spares interchange capability of components and batteries and seating etc. That's all up to you!  E.g. The other day a control pod failed. So I just had a friend fetch me another complete arm with the pod from home to the pub! That's the advantage of interchange-ability. Never even missed a beer. Plug and play.

Where do you get a "cheap" spare Powerchair? Again that's up to you. Craigslist, eBay, newspapers, friends and relatives, the corner shops "for sale" lists, forums like my own & mobility magazines (there are many) and many other places. 

Amazingly its quite possible to pick up some very good used hi-end powerchairs very cheaply!  Just don't be in a hurry.  Another good place is online wheelchair shops or dealers with older chairs.

Online sellers. These mostly push new powerchairs, (because there's a 50% mark-up at least on most) but email or call because they generally have a bricks and mortar business too. Meaning they take old wheelchairs in part exchange. And don't know what to do with them! These often go for very little money. It seems nobody wants a used power wheelchair.