The practical advantage is the port. Two separate wireless devices take two receivers unless they share a protocol; a combo uses one for both, which matters on a laptop or a dock with limited ports. It also fixes the batteries as a matched pair to replace together. The trade-off is choice, since you take the mouse that comes with the keyboard. See keyboards, mice and wireless combos.

Keyboard and mouse combo arranged on a modern office desk with monitor, wrist rest, notebook and tidy workspace accessories.
Keyboard and Mouse Combos

Shop keyboard and mouse combos for offices, schools, home workspaces, desktops and laptops.

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