The stellar.toml file is what you use to publish information about your organization and your Stellar integration. For the complete spec and more information on what it contains, check out SEP-0001.
By setting the homedomain of your Stellar account to the domain that hosts your stellar.toml, you can create a definitive link between this information and that account. Any website can publish Stellar network information, and the stellar.toml is designed to be readable by both humans and machines.
If you are an anchor or issuer, the stellar.toml file serves a very important purpose: it allows you to publish information about your organization and token(s) that help to legitimize your offerings. Clients and exchanges can use this information to decide whether a token should be listed. Fully and truthfully disclosing contact and business information is an essential step in responsible token issuance.
If you are a validator, the stellar.toml file allows you to declare your node(s) to other network participants, which improves discoverability, and contributes to the health and decentralization of the network as a whole.