Some people like to have more control, they may have already some part of the set, and they could have different needs.
The kit is just for "small" lawns. It has just only one exit, so you cannot water differently different part of lawn (e.g. if you want to use part of your lawn for breakfast.
Additionally it has also no control to the "planted part" (flowers, vegetables, trees).
If the kit works for you, use it. But in few years some parts will break. Then you will have the choice: a new kit, or just replacing a part (and possibly with a new model and maker). Maybe you gardening needs increase and you need more complex stuffs.
In any case doesn't trust manufacturer advertising material. It is just advertising, pointing out often nonexistent disadvantages, and pointing the characteristic (without telling you the disadvantages). So check different makers and different kind of irrigation, check the advantage (and disadvantage they wrote from other kinds). Just after that you can see what a specific manufacturer will not tell you. [and you can google, to find people opinions and main problems, but take care: people prefer to write complains, and not success stories, so you should interpret people opinions (they choose wrong kit for they case [hill garden?], just one defective kit (it could happens, but not frequently), bad shopping experience (which affect then the installation and use), "wrong people" (on DIY, some errors, adjustments and being capable to do stuffs is required)).
In any case, if you have hard water, and to prevent icing, some handling is needed also on kits. Sand or dirt in water could requires also additional handling (e.g. works on main pipes few blocks away, and you forgot to deactivate the system).
So if it is your first installation of irrigation system, a kit would help to understand the system and the different components. With time maybe you will revise the needs and maybe you prefer to buy components.