Back before I entered LE and had access to an agency range I didn't often bother to go when it was raining. I did upon occasion, and appreciated the fact that fewer people were using the various outdoor ranges.
After I entered LE work I qualified regardless of the weather. No big deal. I was qualifying in the same weather conditions in which I was working, after all.
Once I became a firearms instructor I spent many years working the range regardless of the weather. I was paid to do so, after all.
Seriously, testing your skills in driving wind, rain, mud, gummy sand and standing pools of water can be a bit illuminating. Having it raining so hard the water is blown upward off the ground or coming at you sideways, coupled with cold temperatures and some wind chill factor, can tell you things about some of your skills under some environmental stress. And again, not to mention being realistic if you're working in the same weather or just out & about in such inclement weather conditions.
Yes, the firearms get wet. Best not to let the wet conditions result in you dropping or tossing your gun into a standing pool of water, though.

Depending on the layout of the particular range, they may also get covered in wet mud and/or sand splashing up or back at the shooter.
Yes, some types of targets do suffer from wind & rain. I've had to brace metal target stand bases so strong wind won't blow the stands over, at least as quickly. Swaying target stands and targets do seem to startle some folks, though, if they aren't expecting a deliberate 'moving target scenario'.
Visibility can change from the perspective that 'hits' on wet targets may not be visible in the same manner as they are on dry targets, too. Personally, I liked this since many of the shooters qualifying were unable to check for their 'hits' (scoring) as they addressed the various courses-of-fire. Instead of trying to obtain 'shot-by-shot feedback', constantly sneeking peeks and trying to make spot adjustments as if it were a 'target range', they had to rely on their skills to address the threat targets throughout the various courses-of-fire. It sometimes made it very apparent when some remediation/corrections were required and some shooter skills may not have been maintained as well as they might.
I used to like bad weather days when our folks decided to await better weather for the increased range time available to me, too, when I wasn't having to qualify or train as many folks.
Now that I'm retired, however, I find I'm not in so much of a hurry to rush out and slog through the inclement weather for some range time at my old agency. I did enough years training & practicing in bad weather that I don't feel quite the same urgency to get up and out in the wind, rain or hail like I used to do when I was getting paid to be there.
I have no doubt I'll be back on some open range in bad weather from time to time at some point, either at my old agency range or somewhere else after we decide where we want to relocate for retirement, because realistic training is still realistic training, regardless of being actively employed or retired.
I can sleep in and have coffee before going out into it from now on, though.
