Every antenna is used both for receiving and sending data. The more antennas you can use, the more data you can transfer at once. That means faster download and upload speeds.
Think of it like lanes on a highway. If you have a four-lane highway, more traffic can flow through at the same time than on a two or one-lane highway.
Going from 1×1 SiSo to 4×4 MiMo means quadrupling maximum data transfer speed. That’s because each antenna supports a separate data stream.