Usefulness = (Functionality x Availability) – Burden
If you will, think of the functionality as what the thing does for you, think of the availability as a number between 0 (for none of the time) and 1 (for all of the time) and think of the burden as what you have to do in order for the thing to function.
An example: A grandfather clock has the same function as a wrist watch - it tells the time, but the wrist watch is more useful. This is because the function of the clock is only available when you can see the clock and as you can’t carry it around you have to be in the same room as it, whereas when you’re wearing the watch you can just look at your wrist and there it is – so the watch is available far more often.
Also the clock has a higher burden; it has a far higher purchase cost, it needs to be wound up regularly, it needs space in which to be stored and it needs to be dusted and polished occasionally, whereas the watch is cheap to buy, the battery lasts for years, it takes up no space and needs very little cleaning.
One at a time.
A watch is one of the many things that you can only use one of at a time. So your first watch will be very useful but a second would not be as useful and a third would be even less useful. It could be said that the availability goes down the more you watches have, so with one watch you could use it 100% of the time, but if you had two watches they would both be available only 50% of the time – presuming you wore them equally often. If you had five watches each would only be available 20% of the time. Of course with five watches you would have five times the burden.
a watch has a back-light then you can see the time in the dark which
is good, but some watches have lots of extra features which are of no
use (unless you're a deep sea diver or a pilot) often these make the
watch difficult to read, add weight and considerable cost.
Like many things, you can spend a lot of money on a watch but it will be less useful than a basic watch as the function will be the same but the burden will be higher.
Don't buy a watch.
Buy something more useful.
How Much Is Too Much?
When the benefit of an additional item is outweighed by its burdens then it is too much.
It follows the formula that: A most useful thing would be multi-functional, reliable, portable, low maintenance and low cost.