What is the grammatical function of 到 in 这些的确是铁匠会用到的老式钉子 ?
用到 = 用得到 where 到 is resultant compliment for the verb 用. 到 here means 'occurred; reached; accomplished' in the sense that without it, it sounds like 'attempted'.
With some other verbs, the differentiation is more prominent, e.g.

抓老鼠 (try to) catch a mouse.
抓到老鼠 (manage to) catch a mouse.

When it comes to 用 the difference is mostly in certainty level but very trivial:

这些的确是铁匠会用的老式钉子
These are true old fashioned clinch nails that a blacksmith might use.
这些的确是铁匠会用到的老式钉子
These are true old fashioned clinch nails that a blacksmith would use.


Side note, 用到 could also mean 'use (...) towards' where 到 is a preposition meaning 'towards; for', as in 把时间用到最重要的事情上. It's unrelated to this question though.