If you don't set a reserved concurrency number, you use the account pool of concurrency which is shared by all lambdas in the account. If one bad / out of control lambda uses all of the account pool other lambdas in the shared concurrency pool can't run. If you set a reservation, that is reserved for that lambda and can run up to that concurrency. It also protects your lambda from being the one that runs out of control (you are capped at that limit)

When using lambda

Lambda

Low

AWS

Mark Andersen