Well, here are a few points:
There ain’t no stealth in space. But there are considerable variations in detectability, from torch drives under thrust radiating at HOLY FUCK K through warm-blooded oxygen-breather ships standing still at 280 K through cold-temperature species ships likewise at 50K through such chill objects as cold k-slugs and so forth.
Things like, for example, idle tactical sensor platforms, or even idle missile platforms, subsisting on a trickle of battery power until an activation signal comes along, are down at the low detectability end of the chart.
If your rules of engagement require knowing what you’re shooting at, or you don’t have unlimited ammunition, you’re going to want to figure out exactly what that just-a-few-K-above-background warm spot is first, cause there’s a whole list of non-military things it could be shooting at several of which violates the Conventions of Civilized Warfare more’n a bit.
Second, just because you’re a warship doesn’t mean you look like a warship. Merchant auxiliaries, Q-ships, the Shadow Fleet, blockade runners, etc., etc., all operate things that aren’t warships to external inspection but may want some of the same information. It’s one thing to hide a military-grade sensor suite from the customs guys, but sending out active pings from your location gives the game away in a way that dropping off some shriekers on the way in in case you need 'em on the way out does not.
Third, everyone shows up on thermal passives, but at long range, that’s a way of saying that you have a big fuzzy blob of heat on your screen. If they’re under way, you can pull quite a bit of information about it’s thrust and mass from the way it behaves, but still, that’s not enough for a detailed profile.
So if you want to establish that that 200,000 ton starship cruising along at 2.2 gravities with the distinct boron line in its drive plume spectrogram is, in fact, the ENS Krazniy Oktaybr and not some other ENS cruiser, or worse, a merchie with a similar conformation, you’re gonna need a hull map.
And last, a close-range ping from active sensors at full power is the galaxy’s universally understood way of yelling “HEY, STUPID!” at another ship. Depending on circumstances, you may not want to do this while simultaneously illuminating yourself.
