In short, it is not a suggestion, but a requirement to add acid to home canned tomatoes. This is for water bath AND pressure canned tomatoes. Why you ask?
Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat. The prevention control in vegetables, meat and other naturally low-acid foods is by heat alone.
Tomatoes can have a natural pH above 4.6 (at least up to 4.8). But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend adding a small amount of acid so they can be treated as a food with a pH less than 4.6 for home canning. Therefore, they are suitable for boiling water canning when the acid is added. (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)
Sources: https://nchfp.uga.edu/publications/nchfp/factsheets/acidifying.html and www.bookstore.ksre.ksu.edu/pubs/MF1185.PDF
