Seeing what a different location sees
The same product page shows different prices in different countries. The same search shows different results. The same advert may not appear at all. If your work depends on knowing what a customer in another country sees, you need to make the request from there, and a proxy is the practical way to do it.
Collecting public data at volume
Any site will rate-limit a single address that asks for thousands of pages. Spreading those requests across many residential addresses keeps each one looking ordinary. This is the largest single use of proxy traffic, and it is why rotating pools are billed by the gigabyte rather than by the address.
Keeping separate things separate
Managing several accounts on one platform from one address links them together, whether or not that was your intention. A stable address per account — a sticky session or a static ISP address — keeps them distinct.
Where a proxy is the wrong tool
It is not privacy software. The proxy operator can see where your requests go, and unencrypted traffic is readable in transit, which is why a reputable provider matters more than a cheap one. It does not make you anonymous, it does not bypass account security, and it does not turn a slow connection into a fast one.
It also cannot rescue a badly behaved script. If your requests look automated, changing address changes how long it takes to be noticed, not whether.
Choosing the type
Residential addresses come from consumer internet providers and are what you want when the far end judges whether you look like a real person. Datacentre addresses are faster and cheaper and fine for targets that do not care. Mobile addresses are the hardest to block and the most expensive. Start with the cheapest type your target accepts, which you find out by testing rather than by reading.