All hit points are useful hit points, and the temporary hit points provided by False life are no different. The spell can be used preemptively before heading into a dangerous area or in the heat of combat as a nice little boost to hold the caster over.
The rules for False Life can be found in the Players Handbook on page 239.
False Life 5e
Necromancy 1st Level
Casting Time: 1 Action
Components: V, S, M (a small amount of alcohol or distilled spirits)
Duration: 1 Hour
Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for the duration.
When you cast this spell using a spell slot of 2nd level or higher, you gain 5 additional temporary hit points for each slot level above 1st.
A simple and straight forward explanation provides the clear benefit to this spell. The caster gains 1d4+4 temporary hit points for the duration. Upcasting this spell is possible though only provides the average for additional temporary hit points.
What does False Life do?
False Life allows the caster to tap into necromantic energy to provide themselves with a number of temporary hit points. Temporary hit points function just as regular hit points except that they cannot be healed and do not stack. They are also consumed before normal hit points.
Is False Life a Good Spell?
False Life is a 1st level spell that grants temporary hit points. It is relatively limited in applicable circumstances and targets only the caster. Ultimately the spell is one of the weaker available to the classes and is probably not worth the spell slot compared to other available choices.
The benefit of the spell is the temporary boost the caster can provide to themselves that might be just enough to get out of trouble or avoid going down for one more round.
Can False Life Stack?
Temporary hit points do not stack. The temporary hit points can be in addition to other features that absorb damage. For example, the Arcane Tradition: Abjurer feature would work with False Life as the feature functions like temporary hit points but doesn’t provide temporary hit points.
If a character has temporary hit points and then is the target of an effect that grants additional temporary hit points, the effects do not stack.
However, the character does get to choose which ones to take. They can keep the current temporary hit points if the total is more than the new ones or can take the new ones if the value would be higher.
False Life and Fiendish Vigor
Fiendish Vigor is a Warlock Invocation that allows the character to cast False Life at will for the additional 1d4+1 hit points. This effectively gives the Warlock an extra 5 temporary hit points at all times unless they are in combat turns.
The character can cast False Life at will, meaning they can just continually cast until they get the max and then hold that for an hour. In combat, however, they would still need to spend an action to cast the spell.
False Life is a low-level spell that really doesn’t hold up well to other spells at the same level. The benefit is the all-around application of reduced damage. More often than not, resistance type spells are going to provide more bang for the buck if the character is on the defensive.
These specialized spells, of course, require the character to know that danger is imminent and what type it is. On the other hand, False Life is a generic catch-all as temporary hit points apply to everything.