Hi there webmasters, we have discussed adding content, photos, htaccess, robots.txt, redirects, server side includes, etc. It's all about adding and adding. Suddenly the server gets filled with all kinds of documents including unused ones (if the bots and crawlers are the only one reading them... there is no need to keep it).
So it's pretty easy to get your mbs growing and growing using more and more space, things get messy. Some webmasters are not familiar with the concept because they build & sell, that's fine. But what about long term websites? when you add content and pictures every day/week/month and then you notice you and your sites have been together for 2, 5 or 10 years, so now you know you are using lots of space and it just doesn't compute.
Why does it matter? space is cheap right? Well nobody wants to face a "I'll buy your site" and suddenly not knowing what matters and what doesn't, worse than that many of us have faced disasters online, be it due to server failures, climate disasters (yes, some of us remember the headlines and big sites going down) even one datacenter burning to the ground.
You don't need apocaliptic events to bring down your site, sometimes updates on your server become a nightmare and you might need to move out. Moving from server to server means time and money, that's when you become grateful for planning and not needing to move 10 gigabytes.
Besides, perhaps space is cheap but speed is everything. So, is your server a mess?. Some CMS allow you to add pictures and files with easy but only few keep track of it and the use of files (file access). You might be surprised but many CMS keep the files even after you deleted it on your wysiwyg, some are more clever and delete every unused file.