1) Higher yields with tiles. Instead of needing to throw away an entire monolithic chip (total loss of that silicon area), you might have several good tiles and one bad tile in the same area (mostly usable silicon area).
2) One design problem doesn't hold up the whole generation. Suppose there is a problem with a new iGPU but the new CPU is performing great. With a monolithic chip, the thing can't ship. With tiles, you can use a previous iGPU combined with your new CPU. This eliminates the need for long delays and the expenses of backporting (like the 26.5 months between Comet Lake and Alder Lake and the necessary costs of creating Rocket Lake).
3) Flexibility. This is related to #2, but you essentially can ship products incrementally when they are ready rather than waiting for all new concepts to be perfected. The CPU team doesn't need to wait for the GPU team to be ready (and vise versa). They can launch what they have and move on to the next project. Being able to have your people work more independently gives your designs, planning, etc far more flexibility. Heck, Intel can even outsource some of the tiles to other companies for even more flexibility.
The drawbacks are of course higher latency, higher power, and/or more costly packaging.