21 gauge staples are commonly used for a variety of lightweight fastening applications, such as:
Attaching paper sheets together.
21 gauge staples are a good size for stapling paper documents, brochures, booklets, etc.
Wire bundling and management.
The small size of 21 gauge staples makes them suitable for bundling and securing small gauge wires.
Craft projects.
21 gauge staples are a popular size for many craft uses like attaching ribbon, string, felt, and other lightweight fabrics.
Picture framing.
21 gauge staples are well-suited for attaching photos, prints, and other documents to foam board or cardboard backing in picture frames.
Packaging.
The small attachment points of 21 gauge staples are useful for sealing up and securing lightweight packaging made of paper, cardboard, plastic, and other thin materials.
Tag attaching.
21 gauge staples can be used to discretely attach tags, labels, and other small items in a semi-permanent fashion.
Screen repairs.
21 gauge staples are sometimes used by screen repair professionals to secure small tears, holes, and seams in window and door screens.
Magazine stapling.
Many magazine publishers and printers use 21 gauge staples to bind and attach pages together during the printing and collating process.
So 21 gauge staples are good for a wide range of light-duty fastening, binding, and attachment needs, especially involving paper, cardboard, craft materials, packaging, and other thin substrates.