In my case (1953 Precision, 1966 slab Precision and 1966 dot Jazz), because I needed the money. There's a case for buying with an eye on what you might get if you need to part with it - however reluctantly. I have a mate who refers to his guitars as his pension.
I paid £950 in 1986 for the '53. How much would that be now? I didn't buy it as an investment but I was aware of its value if I ever did have to sell it. Similarly, I always tell people to buy their gear second-hand because you can always get your money back if you don't get on with it or you want to trade up.