You need minimum one vent and one drain for the hydrotest first, then you might need an automatic drain for removing the condensate (if that's the case) and many other process reasons. Then there is the code which require the manufacturer to provide the minimum number of utility nozzles;- also, every user spec I have seen require vent and drain nozzles, sometime multiple vents and drains. In your case, the air cooler seems to have a small fall towards the outlet header, for the specific purpose of facilitating the venting and draining process.
Cheers,
gr2vessels