I was in Barcelona in June.   There are so many wonderful things to see in the city.   We wanted to go and see the famous Gaudi cathedral Sagrada Familia which is one of the top things to see there.

On arrival at the cathedral there were 2 lines,   one to line up to get 'entry' tickets,  the other to actually go in.   The first line was about 1 hour-long,   and it gave you a ticket for a specific time to enter the cathedral.  In our case it was 3 hours later.    So we had to go away and kill time for 3 hours.   Fortunately we found a lovely cafe and enjoyed some tapas.

To save waiting in the first line for 1 hour we could have booked the tickets online and then just showed up for our 'time slot'.

So if you are planning on visiting any city to see a specific attraction it is definitely worth while to see if you can book your tickets online to save wasting valuable time while you are there.