The first thing I do with a suit set is hold the jacket and trousers up against each other in proper daylight, not shop lighting, because that is where you catch the lie. Cheap sets are cut from the same bolt but dyed in two different batches, and you only notice when you are standing at a till with your hand reaching for your purse and a nagging feeling that the jacket is a shade warmer than the trousers. A good set is dyed as one piece of fabric and cut after, so navy stays navy from shoulder to ankle, and that consistency is the whole point of buying a set rather than mixing a jacket with whatever trousers were nearest.
This is a piece for the parts of the year when you need to look like you have your life together without actually having to think that morning. Work, obviously, but also the awkward middle ground events nobody dresses you for properly: a christening where jeans feel wrong, a work do in a hotel function room, an interview where you want to look serious without looking like you are trying to look serious. I wear mine through fuchsia pink events and navy funerals alike, which tells you it earns its spot in different rooms rather than one. Underneath, the trick is to stop thinking of it as a suit and start thinking of it as two separate pieces that happen to match, because that is what makes it useful the other three hundred days of the year. The trousers go under a jumper in a colour that would terrify most people next to navy, an orange roll neck did it for me last winter and nobody fainted. The jacket goes over a plain white t shirt and jeans on a Saturday, done up on one button, looking like you made an effort you didn't. Trainers soften it, a heel sharpens it, and either way the two pieces stop reading as a suit the second you separate them in your head.
If you have never owned one, start with a colour you will actually reach for on a Tuesday, not the safest navy in the shop but not a colour you will only wear once a year either. A mid grey or a good green does more for you than another black set gathering dust at the back, and a decent weight of fabric, something with a bit of structure that doesn't crease the second you sit down on a bus, will outlast three cheap versions bought in a panic the night before something important.
Would I wear the jacket on its own next week, or is it only going anywhere as a full set?