– The rest period between sets matters if you are looking to optimize your training for a specific goal. – In other words, are the weights light, moderate, heavy, or very heavy?
If you are training to build strength (aka strength training), you should keep your rest periods between 3 and 4 minutes. I'll explain why in a moment.
If you are training for maximal strength (i.e 1 rep max training), you should use long rest periods of about 4 – 5 minutes.