In one of my main areas of interest - Middle East - collecting blocks of 4 is a common format for collectors. Some of the reasons may be to see watermarks and their orientation more clearly (some designs are spread over 4 stamps), to show overprint variations or errors more clearly, without the expense of buying a full sheet, or to acquire a stamp or set of stamps in a format that clearly does not come from a booklet strip.
E.G. Watermark identification:
E.G. Error identification (yes, it is a strip not a block, in this case):
Second stamp down has "crack in wall" error.
It may also be that the stamps are sold as sheetlets of four.
E.G. Saudi Arabia:
Note that the country emblem - crossed swords and palm-tree - is missing from the lower left stamp, and, yes, they are not identical stamps by virtue of the background variations.
Collectors wishing to have selvedge inscription or imprint details may find that collecting blocks of four is an economical way to do this.
E.G. Saudi Arabia, various phrases encouraging the public to use the post-code, clearly address the mail item etc:
To show examples of sheet stamps, rather than booklet stamps which were printed in se-tenant strips.