The 7mm size is most frequently required for specialized automotive and cycling tasks:

: Used for some vintage quill stems, seat posts, and specialized handlebar clamps.

7 mm hex key - old handlebars, cranks & more - Bike Gremlin US

: Occasionally found in high-end or older RTA (ready-to-assemble) furniture that doesn't follow the more common 4mm/5mm standard. Purchase Options

: Essential for removing brake caliper guide pins on many European and domestic vehicles (including BMW, Audi, VW, and GM).

Because they are rarely in standard sets, you will often need to purchase them as individual tools or in specialized "automotive" kits.