The read speed is the rate at which data can be read from your memory card, and the write speed is the rate at which data can be written to your memory card.
Certain bandwidth-demanding tasks can only be performed with cards fast enough to keep up. Tasks like recording real-time HD video, or playing games in-game systems can require faster speeds. It’s best to consult with the manufacturer guidelines for any device you’re intending to use a MicroSD card with to check it’s up to the task.
In general use, file transfers to and from the memory card will always benefit from higher speeds, even if slower cards would be supported too.