I have a protective skin for my phone, a case for my ereader, and a cover that I put over my car in hot weather. Why wouldn't a Mechari want to protect its pretty metal casing from scratches and dirt? If we're going beyond the purely practical, most groups of people like to decorate themselves. Sometimes it's a signal of status or group affiliation (think of a goth versus a sorority girl), and sometimes it's just personal expression. It's easier if these things are fairly temporary and can be changed if the person wants to express themselves differently. Mechari are machines, but in the context of WildStar, they seem to be as much like people as any biological race. Maybe the Mechari is wearing its tier gear in part to show all the other Mechari how accomplished it is, or because it thinks that blue and purple look pretty together.