What are press-fit pins used for?

Yes, they eliminate soldering. The corners of the pins bite into the walls of the plated-through holes with enough pressure to create a reliable gas-tight connection.

There are some PCBs on which the only through-hole devices are connectors and sockets (the rest are SMD, which are reflow soldered), so using press-fit versions of both eliminates an entire soldering step, which saves money in the overall process. Plus, they allow connectors to be on both sides of the board.

Yes, you press it rather than solder it. Decrease the performance? Not so sure, else they wouldn't make them. The old wire wrapping technique was more reliable than soldering, because the two same metals bind together. Soldering is cheaper, but there is one another metal in between (solder) and this can cause galvanic current and a blind joint through the time. Soldering is the cheapest way, but not the most reliable way.