A point of use filter treats water at a single outlet, normally the kitchen tap, rather than at the point the supply enters the building. That is the whole distinction, and it decides both the cost and the limits: you get high grade treatment on the water you drink and cook with, and completely untreated water everywhere else in the house.
Why treating one tap is often correct
Almost all the water a household uses is not drunk. It flushes, washes and irrigates, and treating it to drinking standard is expensive capacity spent on duties that do not need it. Concentrating the treatment at one tap means a much smaller flow, which makes reverse osmosis practical, allows long contact times and fine membranes that would throttle a whole house supply, and keeps cartridge cost proportionate. For contaminants that only matter when swallowed, such as nitrate, arsenic, fluoride and dissolved solids, this is straightforwardly the right architecture.
Where it leaves the problem in place
Anything that acts on the building or on your skin is untouched. Hardness continues to scale the heater, the appliances and the shower, since the softening question is a whole house one. Chlorine still evaporates in the shower and still dries skin and hair. Sediment still reaches every washer and valve in the property, and iron still stains laundry and fixtures. If the complaint is any of those, a point of use filter answers the wrong question no matter how good it is at the tap.
The formats and what each suits
Under sink cartridge systems fit inside the cabinet with their own dedicated tap and give good capacity with easy servicing. Under sink reverse osmosis adds a membrane and a storage tank, is the only common domestic answer to dissolved contaminants, and needs a drain connection and space. Countertop and faucet mounted units need no plumbing and suit rentals. Gravity pitchers are the cheapest entry and the most limited. Our under sink installation guide covers which cabinet arrangements each format needs.
Using both, which is usual
The common real world arrangement is not one or the other. A whole house stage handles what affects the building, typically sediment and chlorine, and often softening where the water is hard, and a point of use stage at the kitchen tap handles what affects what you drink. The whole house stage also protects the point of use cartridges from sediment and chlorine, so they last considerably longer, which recovers part of its own cost. Deciding which you need starts with a water test and a clear statement of the complaint, not with a product.
Questions people ask about point of use filter
Does a point of use filter fix hard water?
No. It treats one tap. Scale forms in the heater, appliances and shower, which are all upstream of it, so softening is a whole house job.
Why not just filter everything to the same standard?
Most household water is flushed or washed with. Treating it all to drinking standard makes reverse osmosis impractical and the cartridge cost disproportionate.
Which format should I choose?
Under sink cartridges for capacity and easy servicing, under sink reverse osmosis for dissolved contaminants, countertop or faucet units where plumbing cannot be altered.
Can I have both whole house and point of use?
That is the usual arrangement. The whole house stage protects the building and also makes the point of use cartridges last considerably longer.