The Supplemental Register is typically available to descriptive trademarks, provided they are actually being used in commerce.
It always makes sense to secure registration on the Supplemental Register when it has not been possible to convince the USPTO that your trademark is not descriptive.
The Supplemental Register provides benefits above and beyond no registration at all.
Although registration on the Supplemental Register does not afford all the benefits of registration on the Principal Register, it does provide the following advantages:
(1) Use of the registration symbol ® with the registered mark in connection with the designated goods and/or services, which provides public notice of the registration and potentially deters third parties from using confusingly similar marks.
(2) Inclusion of the registered mark in the USPTO’s database of registered and pending marks, which will (a) make it easier for third parties to find it in trademark search reports, (b) provide public notice of the registration, and thus (c) potentially deter third parties from using confusingly similar marks.
(3) Use of the registration by a USPTO trademark examining attorney as a bar to registering confusingly similar marks in applications filed by third parties.
(4) Use of the registration as a basis to bring suit for trademark infringement in federal court, which, although more costly than state court, means judges with more trademark experience, often faster adjudications, and the opportunity to seek an injunction, actual damages, and attorneys’ fees and costs.
(5) Use of the registration as a filing basis for a trademark application for registration in certain foreign countries, in accordance with international treaties.
Note that many trademarks for newer businesses start as registrations on the Supplemental Register when the mark describes a feature, purpose, or function of the applied for goods or services. However, continued use and enforcement of a mark over time will allow the mark to “acquire distinctiveness” such that it may qualify for a registration on the Principal Register in the future. Typically, it makes sense to revisit this question when it is time to file your first trademark renewal, 5 years after your mark is registered on the Supplemental Register.