The phrase "are primarily useful" is correct and usable in written English.
You can use it when discussing the main benefits or advantages of something in a specific context.
Example: "These tools are primarily useful for enhancing productivity in the workplace."
Alternatives: "are mainly beneficial" or "are chiefly advantageous".
These instruments employ mechanical linkages and so are primarily useful for measuring static pressures or pressures that change slowly.
"Perpetuating the antiquated idea that women are primarily useful as ornaments, as in the parlance 'Women should be seen but not heard', only adds fuel to the manifold discrimination women still face to this day.
It has two expansion slots: one for postage-stamp-size S.D. (Secure Digital) cards, which are primarily useful for adding memory, and another for Wheat Thin-size cardsct Flash cards, which can serve as modems, wireless networking cards, G.P.S. receivers, and so on.
Since becoming secretary of state, Clinton has told a number of allies in Europe and the Persian Gulf that she is skeptical that diplomacy with Iran will prove fruitful and testified to Congress that negotiations are primarily useful to garner support for "crippling" multilateral sanctions against Iran.
In police departments across Texas, tens of thousands of rape kits have been sitting on the shelves of property storage rooms for years, the result of strained budgets, overworked crime labs and a law enforcement philosophy that rape kits are primarily useful as evidence if a stranger committed the assault.
A: VAT refunds are primarily useful for large shopping purchases.
Water is safe to use on basically every fabric, but is primarily useful just to prevent setting.
That's primarily useful when you want to adjust the type size (most of us will, in fact, be over 40 at some point); you can just spread two fingers apart, as on an iPad, without having to open some settings screen.
The proposed method is primarily useful for structures with a large number of self-similar scales for which using the usual finite element method would be too expensive.
It is primarily useful for a one-time cash infusion, according to Dan O'Brien, a senior analyst at Forrester Research in Cambridge, Mass.
