In general, there’s a lot of variation in current draw from one tube to another, even when the tubes are the same type. This has an impact how they’ll respond in your tube amp — and could cause diminished performance if the amp is designed to work best with tubes of the same current draw.
Some companies offer matched sets of replacement tubes. This means that the tubes have been tested — the plate current draw is measured under normal operating conditions — and each tube in the set has been determined to have the same current draw. This will provide optimal performance in many amplifiers, especially more modern designs. Some manufacturers offer matched tubes with different ratings: “early breakup,” “late breakup,” “hard,” “soft,” and so on, based on the tubes’ current draw, so you can customize the response of your amp.
The first time you install one of these matched sets, you’ll need to bias the amp (assuming your amp is a fixed bias design), but after the amp has been biased to matched tubes with a specific value, you can continue to use tubes from that company with the same value, and you shouldn’t need to re-bias your amp. (It is still wise to have the bias checked on the amp when changing power tubes, to compensate for drift, etc.)
Sticking with matched power tubes can be a good way to quickly change tubes when you don’t want to experiment with different tube manufacturers and vintages, and in most amps will provide the best performance.