the events of requiem are a perfect example. If the infinity didn't have its air power or army the ship would have likely never left requiem. Explorations are dangerous and unpredictable.
Don't forget about Aliens guys Big Scary Aliens, Not to mention that they are after What Infinity set out to find, and more.
Oh and what the artifacts that they are looking for just so happen to belong to The Forerunner WHO HAVE BEEN KNOWN TO HAVE ANTISUNOFABITCHMACHINES AND HOLYSHITTHATSTOUGHIUM ARMOR!
Plus they have Hula-Hoops... Deadly Hula-hoops..