Car rental is not necessary if you're planning on staying in the city centre and don't intend to wonder out of the city to areas around. Driving around the centre can be slower and more stressful than simply taking the metro system which is clean and efficient. The roads in Barcelona can be quite congested and slow moving in the centre.
Car hire in Barcelona may be necessary however if you are on business or have special needs. Barcelona has an excellent metro system that will take you to most of the sights and parking spaces are limited in the centre. You will also find that the roads in Barcelona are often congested and rather hectic, making driving a most unpleasant experience in the city centre.
However hiring a car is a good idea if you're staying in a hotel that is not in the city centre and is not serviced by the metro system.
You could also consider hiring a car if you're intending to leave Barcelona to see the surrounding area. e.g. Girona (ancient town north of the city), Penédes (wine region) or the Dalí museum (Figueres).
Hiring a car may also be advantageous if you want to drive along the coast to visit beach resorts other than the ones in Barcelona. If you don't want to hire a car, but still want to see these sights, you can catch one of the trains that run from Plaça de Catalunya or Barcelona Sants station in Barcelona city centre.
In summary we advise not to hire a car unless you have special needs or will be making regular day trips from the city centre.
see the following page if you want to still hire a car in Barcelona