[Jared Russell]

Of course you don't need one to compete. There are Regional Winners, Einstein teams, and (in many seasons) even World Champions that don't build them.

What a practice bot buys you is (a) extra practice time for your drivers and programmers, (b) the ability to test out and develop improved versions of mechanisms to swap onto your comp bot, and (c) some measure of insurance on your season if after 6 weeks the robot isn't done due to unanticipated design issues, weather, sponsor delays, work travel, etc.

If you design the right robot for the game and for your team's abilities, are able to finish it on time to practice and tune it, and don't encounter any performance issues that you can't fix at an event, you might not need a second robot.

But most (I'd argue all) teams are not good/lucky enough to ensure that all of that happens season after season. For this reason, building a practice robot is one of the first pieces of advice I give to a team that is looking to increase the effectiveness of their robot. It can rescue your season from early mistakes, and even if you nailed it on initial design, your drivers will only get better.