Most companies are not tech companies. They may not have the expertise or ability to make informed procurement decisions. In order to make a decision on (say) which ERP software to buy, they’ll often work with Analysts at firms like Gartner to understand the pros and cons of software in a specific segment. Think of companies like Gartner as a concierge G2 or ProductHunt experience, minus all the spammy crap and with a lot more handholding and support.

The Magic Quadrant (and other similar products) act as a conversation starter and product validation that can give the buyer some confidence that there is some semblance of support, because their ass is on the line if a product they bought turns out to be garbage.

Edit: added additional context