When are train reservations needed?

No and Yes. It is a 38 min ride to Girona and the trains going that way come every 10 mins.
The train to Figueres is 1 every hour

Generally, the ONLY reason to reserve trains in advance is to take advantage of any discount that could be available. However, all discount tickets come with restrictions. So take that into account when deciding to purchase advance tickets.

I agree there is value to book ahead for savings, but for short routes I personally don't bother. Some short runs have savings, but if so, it is rarely enough for me to have to micromanage my schedule before I arrive in country to determine an arrival time.
Now, there is some savings to buy a return ticket, but then you are still somewhat locked in to a set return time....and may only want to book a few days before, or morning of travel. Keep in mind, however, that sometimes trains in the evening sell out as it might be a commuter run, so do be aware of this and determine your option or whether returning by bus is palatable.

Both routes can be done by Regional and Interregional trains that don't have seat assignments and therefore no authority judges them "full." But if you choose a departure in the AVE/Arco/Alvia/Avant family, then tickets will only be sold for specific dates, times, and seat assignments, even at the last minute, or would require a seat reservation with a rail pass, and these trains can possibly sell out.