When considering what kind of power supply to purchase there are a lot of things to consider. There's the efficiency rating, power supply size, and even wattage. The thing that is not often discussed is modularity and why it would be useful. Let's talk about it.
Why would I buy a modular power supply?
The primary reason you would buy a modular power supply over a non-modular is cable management. Some cases have very little room to route cables and as a result, you need to be picky with which cables you need vs which ones you do not. For modular power supplies, the cables are not grouped together and can individually be routed to wherever they need to be. Below is what a modular power supply looks like. Generally, there is no real drawback to modular power designs, however, they can be expensive due to their unique designs but still are usually the preferred PSU type.
A semi-modular power supply is one that combines both words. The main connections like the ATX 12v and the 24pin are connected to the PSU and grouped together, because typically all systems need at least those two connections. The image below shows a semi-modular power supply. The advantage to this is ideally the same as modular, you are only using cables that are immediately needed. The extra cables can be omitted depending on what hardware you have. They can also be more affordable than fully modular PSUs. The con to this type of PSU is that routing the cables for things like the 24-pin is more difficult because they're bundled.
Non-modular power supplies have all the cables tied to the main power unit. The picture below shows how all the cables terminate into the main block. Because all the cables are tied to a single point, the primary drawback to this PSU type is its routing. It is harder because the connections all have to be routed near each other with less flexibility. That being said, non-modular power supplies are very affordable and sometimes have the same reliability rating as high-end modular power supplies. They are useful in budget situations or in computers that don't need to hide the cables as much.