Over time, disk drives accumulate junk files and these consume valuable space that could be used for more useful purposes. The junk files are not obvious and you won't find them easily in Finder windows, so it is not easy to keep a disk clean and healthy. However, there are utilities, such as CleanMyDrive, that makes keeping a disk clean a very simple process.
Disk drives come in a variety of sizes and some Apple Macs have quite small drives. If you have an SSD (solid state disk), it is likely to have quite a limited capacity for storing files. Older Macs have small disk drives, even if they are traditional mechanical models. Anyone that has a small disk drive, or even a big disk with lots of files, will need to keep an eye on the space used and the remaining free space.
How soon after starting your Mac can you begin to use it? Why is it so slow? What is it doing? Is it OK to turn off or restart? OS X works hard behind the scenes on a variety of tasks, but you would never know because often nothing is displayed on the screen.
A question posted in an Apple help forum was "How do I clean my Mac's disk drive?" The reply was "You don't need to do that on a Mac." It is one of the many myths surrounding the Mac and although it may be better than some operating systems, it certainly does need cleaning occasionally to get rid of the file clutter.