The RAM is the memory used to store programs while the system is running as it is the fastest memory your system has access to.

The amount of that memory is key when having multiple programs opened at the same time.
You may also like to have dozen tabs open in your web browser for example. Well that needs a lot of RAM.

RAM is a quick buffer to write/read data (information) and your Mac is constantly reading and writing data.
With a low amount of RAM, your Mac will have to use its storage device, which is slower and sometimes way slower.
If that storage is a hard disk (aka mechanical spinning disk) then you’ll be up for long waits.

Note that RAM won’t help when it comes to reduce load times at start-up. That’s the job of the SSD…

As we’ve explained earlier, when your Mac has to frequently use the storage because of a lack of RAM, your Mac will become slower.

If your actual storage is a hard drive (also called hard disk), then your Mac will suffer serious delays.
Hard drives are way too slow and have other drawbacks like power consumption, not being shock resistant and slow access times.
SSDs (using Flash memory) came up more recently and became the storage of choice for MacBooks since 2012. That technology finally became more affordable and reliable over time.
Since Apple was still selling you iMacs with hard drives even in 2019, we saw plenty of confused customers wondering why their new iMac was so slow after less than a year.
Don’t look any further, if your Mac has a hard disk, that is the reason.

SSD upgrades will speed up boot times, program load up times and overall, the responsiveness of your machine.