Question about BN for GBA
 

I got Battle Nexus for GBA and I cant figure these things out
A) What is the point of finishing the bonus tracks in the race mode?
B) What is the point of finishing the bonus levels in battle mode?
C) What is the point of getting all the crystals? You dont seem to get anything I did it for the first area.
D) What is the point of beating the level with all 4 turtles?

Thank You to anyone who can answer these

I didn't bother with the first 2 but I can help you with the last 2. I have only played the game on the hardest setting so I'm not sure if this applies to the easier settings.

If you don't get all of the crystals you can't get to the last stage. You need to use more than 1 turtle per stage in order to get some of the crystals. Some of their abilities allow them to reach crystals that other can't.

Oh alright thank you! That helps haha. I have been collecting all the crystals but I wasnt going to bother if it was pointless