You’re comparing two VERY different things.I opt to back up to my Mac over iCloud for several reasons. First, I don’t want my personal data sitting on a company’s servers if I can help it. Second, iCloud doesn’t do versioned backups, whereas a Mac backed up with Time Machine does (although iOS/iPadOS makes it difficult to retrieve data). Third, I don’t want to be at the whims of a company that has the only other copy of my data. To me, that’s asking to be bitten. Fourth, backing up to my Mac is almost as easy and about as fail safe as iCloud—iOS devices back up to my Mac automatically daily, and I switch out the Time Machine drive with a duplicate drive I keep at work weekly. It takes a few seconds per week.
Also it’s not 99 cents unless you plan to use your device for only one month.
Making an iTunes backup of your iPhone definitely doesn’t clear space off of it.
You can’t say: “Wow, I’m using 127gb of 128gb on my iPad.... I should back it up now, so I’m good for another few years”... however, you certainly could turn on iCloud everything, free up many gigs, & keep rockin & rollin.