Hello. It will be risky just turning up at the station prior to departure time. It is not like UK where once you have a ticket you can get on a train, even if you have to stand all the way.
Your pre-booking will mean you have a seat on a specific train.
The cost from L'Aldea is about €47 single. You can also do the journey by coach. ALSA do tarragona - murcia for €44 but it is an overnighter