One of the main reasons why people look at USB to SPI options is the fact that there is no need to learn low level SPI protocol. This is a major positive for people who are trying to make everything as easy as possible to use when they are moving everything around.
Any SPI chip can connect to a USB to SPI adapter. It really just comes down to using what is best for a person individually. There are some pretty cheap options online, and most people are going to go that route if possible.
Those who are a little bit pressed for time can also find these at a local store. Search around and ask for a good deal if possible. Chances are it is going to be a little bit more expensive, but not too much that it is a major hindrance at all.