Why these run in your browser
Every tool in this category processes files locally. Your data is read into memory, transformed, and written back out as a download — it is never transmitted anywhere. That means no upload wait, no queue, no file sitting on someone else's disk afterwards, and no account to create.
It also means these tools are bound by your device rather than a server. Processing speed tracks your CPU, and very large files are limited by available memory rather than an upload cap. Where a tool has a real limitation — a format it cannot reach, a quality trade-off it has to make — it says so in the interface before you commit to a run, rather than after.