You can use either Interfolio or AADSAS to send your letters to dental schools.
Interfolio is not necessary, but it's useful if you have a lot of letter of recommendations and documents.
Interfolio is not useless. It stores any confidential letters or things you want to sent to companies, processing centers, etc. You also have the option to renew your interfolio account (with a fee) and have the letters stored in case you need to reapply. AADSAS deletes all letters after the cycle is over.
However, you have to request the letter first. You send out an email on Interfolio to your evaluator by putting in his/her email. The evaluator then uploads the document in his/her side. Once your evaluators upload the LOR onto Interfolio, you click around on Interfolio to request your LOR and Matching from to be sent from Interfolio to AADSAS. There is a fee for each document you send, ~$6?
On AADSAS:
You choose "paper" option on AADSAS, put in 1 entry per LOR.
Print out the matching form. Scan it. You upload the matching form to Interfolio yourself, and request to send to AADSAS the letter of recommendation with the AADSAS Matching Form.