1st class seat reservations are often also more expensive. Without a seat reservation, you cannot travel in that train. You can book 2nd class seat reservations, when having a 1st class pass.

Not in all countries though. Sometimes you can get seat reservations for free (Like Austria and Norway) when having a 1st class pass).

Seat reservations are optional on most European trains. As Brendan said, you can get them for free in most countries.

However this is not the case on high-speed trains in France, Italy and some cross-border routes (+ pretty much all Spanish trains) : you need a seat reservation, otherwise you won't be allowed to board.

General advice for seat reservations :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

https://www.seat61.com/interrail-and-eurail-reservations.htm

The TGV between Barcelona and France is very popular so you need to book well in advance. It has one of the most expensive seat reservations unfortunately but regular tickets often cost 200€ (so the pass is still good value)...

To save money (but not time) you can take the coastal route via Portbou to Perpignan, Narbonne or Montpellier. Fully reservation-free. From either city take a TGV to Lyon (in both classes : limited 10€ fare, then 20€ until the train is full).

Book TGVs within France here (clearer overview, less booking fee) : https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

“Do I really need to pay an extra to travel in 1st class?” - Yes, you must pay extra to reserve a seat on a train that requires a reserved seat, such as the one you have selected.

“Do I really need to pay an extra to travel in 1st class?” - No, you can travel on thousands of trains that do not require seat reservations without paying a cent/penny/pfennig/centime more.

We got bumped from a train with first class tickets for not paying the extra to reserve seats. We have not been able to sit in first class at all after being here for 3 days due to all seats being reserved. We’ve gotten bumped out of seats at connection stops as seats get reserved while you are enroute .  We had to sleep on the ground with our children for the night  in a small village until next morning train due to no more trains.