Despite being the sixth round of the season, the Spanish Grand Prix was really the first chance to test the cars' all-round performance, as the Circuit de Catalunya, with its classic layout, is the most visited test venue for the Formula One teams.
Every part of the car needs to deliver in order to run a good lap of the track. It has long corners, long straight, slower sequences, and an abrasive surface to test tyres. With this kind of layout, teams with good pace at Barcelona should also go well at most circuits, but especially at places like Suzuka and Silverstone.
Added to this, the weather for the race is normally hot, which also tests the car's cooling and again its tyres. However, despite the huge amount of mileage the teams do at this track, the majority of testing is done over the winter and the temperatures are much lower, so it's not unusual for runs in the morning to be delayed due to frost or fog.
Moreover, the Barcelona track is very dependant on wind direction. This particularly affects the long straight and fast corners, with head winds increasing downforce/drag, slowing the cars on the straight, while tail winds rob the cars of downforce, affecting them in the fast corners.