Arnold and other "early" body builders were obsessed with the pump. Their theory was that with all the blood pumping into the muscles, they would stretch and have more room to grow. I've read some advise advocating trying to get a pump with every exercise.
Now, there may be some truth in the matter -- Arnold and his competition had some of the best physiques ever, in my opinion. That being said, in more recent years it's been proven that pump =/= growth. Training geared toward strength, for example, will experience a lot less "pump" than training for hypertrophy. However, to suggest that strength training will not make you bigger and stronger is obviously inaccurate.
Ultimately it comes down to personal preference. As others have said, diet and preworkout supplementation have a huge impact on the pump you experience. Certain oral steroids and prohormones have notorious pump-boosting effects -- to the point where calf and back pumps make cardio painful if not impossible. If you enjoy the pump, perhaps changing your training schedule will enable you to experience it more frequently. However, your progress is not dependent on the pump, so if morning workouts are more convenient, then stick with those.