Placebo for some and some are very happy with it.
I got one because I shot a lot in my club and I am taking out a liner for cleaning more frequently, so for my own convenience only - not to grease my fingers all the time with oring lube.
Also I don't live in a hot place where the liner original oring material softens much with a day heat, I never had problem with POI earlier without, and now with the liner sleeve I could not see a mentionable difference in POI or grouping precision or consistency.
Many people in hot South find the stiffening a liner very useful.
UAE, how is a weather at your place?