If you are planning on staying in Barcelona, then hiring a car will be unnecessary unless you need it for business or you have some special needs that only a car can accommodate.
If these don’t apply to you, then you will find that the metro is far less stressful, and is a safe, clean and efficient mode of transport. Alternatively, you could also get a taxi, which is more expensive than the metro but isn’t extortionate.
If you are not convinced and think that hiring a car is for you, then remember that much of the centre of Barcelona is pedestrianised, and the roads that you can drive on tend to congested and move slowly.
If you are planning on exploring further afield such as to Girona, Penédes or Figueres, then car hire could be useful, and there are many car hire companies that you can use, either in the city itself, or online before you make your trip.