why? with competitor,
first, team will push their limit to keep their customer not going to another product.
second, team will get motivated from how their competitor challenging.
third, team can get inspired tools/tech from competitor.
fourth, team will offer their best services for their customer to make their costumer not going away to another product
fifth, team will continue to improve by many mean. ( i hope my english for this one is correct 
)
if team doesn’t have any competitor, its so bad, less motivation, less inspiration, need more RnD to know what customer need. market will be “quiet” and didn’t have any “lust” to do this, do that 
like us too, sometime we get inspiration, motivation from another product (games) so we can create more more game, what happen if no one creating RPG game, how we can get ideas or motivation?
just my opinion