* To show pride in their local area. The M25 is a major motorway in England that circles London. People who live in the surrounding area might put an M25 ring sticker on their car to show their pride in their local community.
* To show support for a local football club. The M25 ring sticker was originally sold for a charity fundraiser supporting local football clubs. It's one of several ring stickers associated with clubs who were in the original first division when it broke away to establish the Premier League. The M25 is an outer orbital motorway circling London, and these clubs are all London clubs so the badge could have significance due to its circular shape but this isn't confirmed from the creator/distributors. People who support these football clubs might put an M25 ring sticker on their car to show their support.
* To simply decorate their car. Some people might put an M25 ring sticker on their car simply because they like the way it looks. They might not have any particular connection to the M25 or the surrounding area, but they might just think the sticker looks cool.
Ultimately, the reason why someone has an M25 ring sticker on their car is personal to them.
