Publishers help by providing financial means, recording demos, finding producers and buying instruments and recording equipment. They can help finance tours, video clips, websites and marketing for your work.
They help you develop your catalogue, by setting up meetings and finding collaborators. They offer your work to other artists for sampling or remixing.
A remixer is an arranger. Remixers negotiate a fee for the remix master as well as acting as publishers. Publishers receive no money from the remix master, which goes to the producer and the artist, but does receive royalties for the work.
Publishers play a central role in identifying synchronisation sessions and requests for music for moving image productions.
As an artist or record label, you may want to work with a publisher who has the know-how to promote your catalogue. You will make more money from your work than you would without their help. You will benefit from your publisher's skills and their network of professional contacts in music, cinema, advertising, video games, and the internet.
More generally, as with other types of music, publishers are there to develop your career. The publishing revenues they receive enable them to help you acquire equipment for music production, book studios, invest in marketing, etc.