Language is not merely a vocabulary and a set of rules for how to generate and parse well-formed sentences. A proficient speaker also knows how to express himself idiomatically and in a culturally appropriate way. It also provides metaphors which can help a speaker express ideas.
A language is not a question of belief - normally, a speaker doesn't have to "believe" that 'nutmeg' is a spice derived from a certain plant's grated nuts. It's a question of convention. A speaker doesn't "believe" that it's rude to say certain types of utterances in certain contexts, it's rude by his understanding of the conventions of the speaker community.
In some ways, religion provides a similar set of conventions. Sometimes, some of these conventions may seem odd and superstitious, but as a shared set of conventions, they may help provide some way in which to parse human interactions, as well as render them somewhat more constructively predictable for other participants in the interaction.
Let's consider one example - sitting shiva. In Jewish culture, upon someone's death, there are many rules that govern the behavior of the nearest family of the departed, as well as how the community is expected to behave with regards to them. After the funeral, shiva begins and lasts for a week. During this week,
- You are expected to visit those who grieve.
- You don't start a conversation with them; you let the grieving family start conversations if they like.
- You can bring food. (NB: Of course, this is expected to be kosher food.)
There are more rules than these - and the rules may seem onerous and nitpicky. But ...
- They provide a clear 'manuscript' which to follow; you don't need to worry about what to say to the bereaved.
- They provide a clear, understood way of how to express your sympathies. You know what to do, the bereaved know what you'll do. This is a bit like having a shared set of vocabulary, and using that shared set of vocabulary. You don't need to come up with your own attempts at showing sympathy, and risk it being misunderstood.
In Jewish mourning, there's several other minor rituals - some of which are somewhat physical (and maybe cathartic for that reason, e.g. tearing a cloth,
Sitting shiva has a very specific time frame - a week. Other parts of the mourning last for a month, and finally some parts a year. This might seem odd to regulate, but this too provides people a 'manuscript' to go by. No matter how important a person was to you, life must go on. Providing rituals and specific dates as a kind of roadmap to grief may in some sense help manage the emotional baggage.
Probably, other practices besides funerary one provide similar contexts to people - even less dramatic ones like, say, festive holidays. These provide a shared language for joyous interactions, without having to come up with a great deal of explanations why I'd like to get someone drunk.
