Difference's between album versions vs single versions

[MacShadowsBall]

I'm intrigued by the differences between an album version of a song vs the single versions of the same song. Perhaps its more of a generational thing growing up in the 90's, but I don't get why you change up/remix the song for the single release. What's the reasoning in changing the song up for the release of a single?

Take "Say You Love Me" - 2 versions album version and the single 45 version. Though I do prefer the single version of SYLM to the album version.

I suppose other artist do/have done this, but it just seems strange to me.

Are there FM songs where you prefer the single version over the album version or vice versa?