ATX or Advanced Technology eXtended is part of an Intel-developed platform which follows on from the older AT standard to allow for better design, airflow, standardization of components and ease of installation for the end user while offering backward compatibility with the AT format.
The ATX standard encompasses several things, including:
The key benefit is that it allows you to be sure that when you purchase computer parts such as memory, power supplies, graphics cards, or even a motherboard, they will be compatible.
As an example, with a modern ATX motherboard, you can purchase an M.2 drive and be sure that it will fit without issue.
Whereas, without this standardization you may find a situation where you purchase a M.2 only to find it does not fit the standardised connection or slot.
ATX is an evolving standard, with the latest revision ATX 3.1 focusing on power and reliability and introducing the updated 12V-2x6 connector, which, while it has proven controversial, it is an evolution which allows for higher power graphics cards it allows for them to have up to 600W through one connector as opposed to needing four PCIE 8-pin cables to achieve the same output.
Within the standardization there is still a great deal of room for innovation and improvement, from features such as toolless M.2 drives, or component manufacturers improving mounting hardware for coolers.