A good installer will match slabs, when necessary, and account for unsquare walls, plus, custom finish the edges and sink holes, etc to your needs and desires. Plus, you generally get to pick out the slab(s) you prefer from a huge variety of stone. Most custom stuff is 3Cm thick. This can be critical when you need more than one piece, but not so much if you can do it with one.
The off-the-shelf stuff often is thinner at 2Cm, which requires a bit more support, is often only available in a few stone types, and you may not have the option of a sink hole, or if you do, it will be for a specific sink, and you may not like the sink. Fabricating a sink hole on-site is possible, but expensive and subject to cracking the slab. Often, you end up with a drop-in sink verses an under-mount, since it is really hard to fabricate proper anchoring for an under-mount in the field. Matched pieces and joints are difficult with the pre-fabbed stuff, if you need to turn a corner or use more than one slab. Cutting a piece in the field can be done, but it won't look good if you need to have a finished edge on it unless you have specialized tools, and some skill.