If you are shooting on a budget, many desirable effects are tantamount to impossible to guarantee, particularly if you want to capture them in camera. Rain is one of the trickiest and with this tool, you can add it in, realistically, in post.
Rain is a brilliant weather condition for creating a mood in video or photography, but it's a long way from an easy inclusion. Even if you lived somewhere with monsoon season and could all but guarantee heavy rain when you wanted to shoot, a production doesn't really want rain as it's so difficult to control. The ideal setting is milder weather in which you create the rain yourself.
This is again tremendously difficult to do convincingly. I've had to do it a few times and even with a purpose-made hose for creating a large rain effect, it's awkward to make it seem realistic and large enough unless you have a long lens attached and a tight crop on the subject. A wide-angle lens makes it almost impossible. These sorts of arrangements also require more crew and a lot of care, so many opt for visual effects.
In this video, Film Riot shows how strong some post-production effects can be in 2022, with ActionVFX able to add in a torrential and convincing downpour without a drop of water shed.