Craft kits by hobby
A boxed set is the obvious first purchase and it is not always the right one. Sometimes it is clearly better value than the same items bought individually, and sometimes it is a tidy box containing four tools you wanted and eight you did not. The difference is predictable.
When a set is worth it
When you do not yet know what you need. If you are starting something new, a set is the cheapest way to find out which tools you actually reach for. That information is worth real money, and you cannot get it by reading.
When the set contains a handle you would buy anyway. Handles are the expensive part and blades are cheap. A set built around a handle you want is usually cheaper than that handle plus the blades separately.
When it includes something awkward to buy alone. A mitre box, a saw and the handle that fits it are a bad set of three separate purchases and a good boxed set.
When it is a gift. Obvious, but it is the case where the box itself is part of the value.
When it is not
When you already own the handle. Most sets are a handle plus an assortment. If the handle is already on your bench, you are buying it twice to get blades you could have bought outright.
When you know exactly what you need. Two specific blades and a handle will be cheaper than a twelve-piece set containing them.
When most of the set is for a different job. A carving set bought for one gouge is an expensive gouge.
Which set for which work
Model kits and miniatures. A hobby knife set built around a light handle and an assortment of fine blades covers plastic kits, decals and detail work. Pair it with a cutting mat and tweezers and the bench is complete.
Model railway and scenery. Scenery work needs cuts a knife cannot make: mitres in strip wood, square ends on balsa. The ship modellers set and the mitre box set both cover that ground, the second being the cheaper way in if a mitre box is the only thing you are missing.
Woodwork and carving. The woodworking set is the general answer. If you specifically want to carve, the deluxe woodcarving set is the more focused purchase.
General making and repair. The builders tool set and the knife and tool set both spread across a range of jobs rather than going deep into one, which is what you want if the work varies week to week.
Starting from nothing. A 15-piece craft and model tool set is a reasonable first box for someone with no tools at all.
Two things to check before you buy
What handle is in it. The handle determines every blade you can fit afterwards, and a set built on a light-duty handle will never take heavy-duty blades. That is the single most consequential thing in the box and it is rarely the thing highlighted on the front.
Whether the blades are replaceable singly. Almost always yes with Excel, which is what makes these sets worth owning: when the assortment runs out you buy the two blades you used rather than another set.
The compatibility chart on our Excel page shows which blades fit which handles, so you can see what a set commits you to before you buy it.
The short version
Buy a set when you are starting out or when it contains a handle you want. Buy singly when you know what you need. And whichever you do, buy more blades than you think, because that is the part everyone underestimates and the part that decides how the work comes out.