Why would someone buy a standard USP and convert it for single action only?
The HK USP can be configured to a number of different variants with respect to trigger, action and lockwork. Variation 1 and 2 permit carriage of the weapon in a single action configuration, but also permit a double action pull. Variation 1 permits use of the safety lever for decocking, enabling safety in single and double action, or decocking after any shot.
In any event, these variants are not single action only, and may be carried double action or single action.
The USP comes in double-action-only in standard full trigger pull, or the LEM trigger modification.
It does not, however, have a single-action-only variation. It can be carried in a "cocked and locked" mode, but is not available in only single action.
However...addressing the question as to why someone might prefer single action only...a number of shooters, myself included, find a single action only pistol to be an excellent choice. I'm a long time 1911 shooter myself...and while my personal daily choice is a Sig P239 with either a HKP2000SK or G27 backup in an ankle holster, I still hold a soft spot and a strong preference for a single action pistol.
I have an HK USPC in .45 with a V1 trigger, capable of single action carry, which is a double action-single action configuration, manual safety, and decocker. Even though it's capable of single action carry, I don't operate it that way, and it gets carried hammer down on a live round, for a first double action pull. My P2000SK, conversely is hammer down on a chambered round, with an LEM trigger, and as a result, no manual safety. No issues either way...both comfortable means of carry. I'm more than happy to carry a 1911 in "condition one," cocked and locked, and I do.
To each his own. The USP can be carried either way in Variant 1...but it's still not a single action only handgun, even if carried in a condition 1, cocked and locked, mode.