Watt do you need?
What kind of power supply you need is of course different for every system and depends on the components you use as well as whether or not you are overclocking the processor and graphics card(s). The easiest way to get a reasonable indication of the power consumption of your specific system is by using an online Power Supply Calculator. Multiple power supply manufacturers offer these tools; Cooler Master and Seasonic offer extensive and user friendly variants, among others.
If you are building a high-end gaming system based on an Intel Core i7 8700K and a single GeForce GTX 1080 Ti card, an SSD and a hard disk, you will see that the power consumption of such a system will end up at about (a maximum of) 450 watts. A power supply of 500 or 550 watts is suited for this. However, if you want to upgrade to, for example, an SLI-setup in the future, the estimated consumption increases to more than 710 watts, which means that you need a power supply of at least 750 watts.
If you are a content creator, or someone that is often busy with photo- or video editing software, or someone that does 3d-modelling, you might choose an Intel Skylake-X or AMD Threadripper processor. In combination with a fast graphics card this leads to a considerably higher power consumption than a 'mid-range' processor like the i7 8700K, which means choosing a power supply that has a capacity of more than 600 watts is a good idea. In short: about time we tested a good amount of heavier power supplies.