Buying for a modeller blind is close to a lottery. The kit you pick may already be waiting on their shelf, the paint range they use may not be the one you chose, and the tool they actually need is probably something you have never heard of. A gift voucher steps around all of that: you decide the value, the person who receives it decides what it turns into.
Vouchers can be spent anywhere in the catalogue, from kits and detail sets to paint, tools and books. The terms of validity and the way a voucher is redeemed are stated with the voucher itself.
Experienced builders most of all. They have settled habits, a preferred paint system and usually a shelf of half-built projects, so a specific kit is a risk. The same logic applies at the other end: with a complete beginner you rarely know yet which direction will stick, and armour, aircraft and ships each need different equipment. A voucher also works when you know the budget and nothing else.
Then start with new releases. What has recently entered the catalogue is a fair hint at what the hobby is talking about, and it is easier to judge a subject there than from a shelf list. A voucher remains the safer option whenever you are unsure of the scale they build in.