Bars are loud. So are concerts. Being able to communicate without screaming at the bartender or your friend can be very useful. Just signing "bathroom" from across the room can help you communicate to the person you're with where you are going without much effort.

Someone is too far away to hear you. Making gestures is already something people do but they are not practiced in this so when people are out of their speaking range sign language codifies these gestures and allows people to quickly communicate from long distances. This could help when someone is on the other side of a room or in the street and you want to meet at a third destination.

You're in a post-apocalyptic scenario where you need to be quiet. ie: zombies, A Quiet Place, The Last of Us. When making noise is deadly then sign language seems more like a necessity than a suggestion. A lot of these stories include sign language, and I think that this is very thoughtful and practical for these stories to include. When things go to shit or you need to be sneaky sign language is a great tool.

Babies can start to communicate gesturally before verbally. This allows you to communicate with your child before they can speak. This helps them not feel as frustrated when they are needing something and allows them to internalize language earlier than with spoken language. While not everyone has children I think being able to communicate more effectively is good for everyone.

You want to have secret conversations in front of others. As long as they don't know the language. You could create variations or unique signs just for your inner circle. Use at your own risk if others recognize what you are saying. The more people that know the language the harder it is to be sneaky but lucky for us now most people don't know any sign language so this will allow for sneakiness for quite some time.

You are in a tactical scenario where the enemy might be able to hear you but you need to communicate with your team. The military uses sign language in some capacity so it obviously works for this type of scenario.

You can communicate at night and not wake someone else. For example, if you're on a bus or plane and everyone is sleeping.

Your hearing starts to get worse as you get older or your hearing gets damaged. Maybe your hearing aid battery died, no problem, you know sign language.

You can talk shit in front of people, if they don't know sign language. Use at your own risk. People can oftentimes read body language and can tell when someone is talking about them so it is better to act nonchalantly and not direct others toward your sneaky shit talking.

Basic sign language is intuitive and easier to learn than most other languages. It is expressive and teaches you to focus on the nonverbal aspects of communication in a unique way. It helps you express yourself in a new way and understand what others are expressing with their bodies and faces.

Bonus: You can communicate with deaf people! This seems obvious but being able to communicate with more people in your community has many benefits and allows you to include more people into your world as well as allowing you to learn more about another culture.

Basic sign resources:

Lifeprint.com

https://www.lifeprint.com/

20+ Basic Sign Language Phrases for Beginners | ASL

https://youtu.be/v1desDduz5M

American Sign Language Alphabet