There are no Eurail trains, the coverage works by train companies and the vast majority of European trains are included.
I guess you looked at the map on the website ? Unfortunately it is very incomplete and doesn't show half the routes possible.
Where are you looking to travel ? Seat reservations cost extra and limit your flexibility in some countries : mainly France, Spain and Italy + some cross-border routes. Some of those routes are really popular and need to be booked well in advance (thinking of Eurostar for example).
Regular tickets wouldn't really help as they also become more expensive closer to travel date.
On the other hand most of the time you can literally decide seconds before boarding which train you'd like to take! Eurail gives you that flexibility.
I'd suggest reading these 2 links first :
https://www.seat61.com/how-to-use-a-eurail-pass.htm
https://www.seat61.com/interrail-and-eurail-reservations.htm