Most PDF tasks are small and annoying. Two signed pages need to become one document. A 40-page report needs to lose its appendix. A scan came out sideways. A file is 2 MB too large for a submission portal. None of it is hard, and none of it should require an account, a subscription, or handing a confidential document to a company you have never heard of.
That last part is the reason these tools work the way they do. Almost every free PDF site online uploads your file to a server, does the work there, and sends it back. For a holiday photo that is fine. For a signed contract, a payroll summary or a client's financial statements, you have just transmitted confidential material to a third party and accepted whatever their retention policy happens to say.
These tools do the work in your browser instead. The file is read from disk into memory on your own device, modified there, and written back out. It never travels. A useful side effect is that there is no upload wait and no file size ceiling imposed by a server — the practical limit is your own machine's memory.