There are a lot of benefits of having golf clubs from the same brand in your bag.
Blending between models: It will come as no surprise that all the manufacturers will want you to play as many of their clubs as possible and make this decision as easy as posible by matching the lofts, length, and lie angles up between sets.
For example, suppose you have a set of Callaway irons, which get shorter by half an inch, and are in the market for some new wedges. In that case, Callaway wedges will continue this pattern to match into your bag far more straightforwardly than another brand. It will be the same for the loft and lie angles, making the transition as simple as possible and avoiding distance gapping.
Like for like: Another benefit is how easy the decision-making process can be. Although new golf clubs is a big purchase and you'll have done all the research required, sometimes familiarity reigns and you want a more recent version of what you currently have, and the brands realise this.
If you want a new TaylorMade driver with a draw bias (as that's what you have currently), there will be a more recent model with the same benefits.
It's the same in irons. For example, if you had a set of Titleist 712 CB irons, you'd be able to replace them with a set of 720 CB irons, which will look and feel much the same but bringing all the benefits new golf clubs have in terms of technology.
Shaft adapters: For better players and those who get custom-fitted, there is continuity through the years in the shaft adapters fitting the newer heads. If you purchased an aftermarket shaft and want to keep it with your new driver (for example), it will just slot straight in.