When working with large blendfiles, then compressing maybe handy in terms of drive space. After clicking the 'Save as' or 'Save copy' option, I see an option of checking 'Compress'. Why is this not checked by default?
Blender developer and graphics software engineer Brecht Van Lommel answered this question on the developer.blender.org website in December 2018 (https://developer.blender.org/T54943). Here's the exact question and quote:
Is there a technical reason about why "Compress" option is not enabled by default when you save a .blend file? Slow hardware maybe? I do not think people can use very slow computers with Blender 2.8 anyway.
This option makes file saving and loading significantly slower. With a faster compression algorithm it could be enabled perhaps, but right now it's not good enough.
Another downside is that if you are using a version control system, it will actually increase file size.
So, if you're not using a version control system and aren't troubled by the performance hit, you should be fine to enable the option. Alternatively, you could simply compress your .blend files for archival purposes (within Blender or otherwise) when you're not actively working on them.
I've heard Ton Rosendaal (the original creator of Blender and chairman of the Blender Foundation) speak of how important it was for Blender to be extremely performant and responsive - and when you consider how much time it takes for many software applications to simply start up (consider almost anything in the Adobe suite, for example), it's clear from Blender's generally excellent performance that this has been a guiding philosophy that has been respected and adhered to for some time.