Griddles are not usually sold with grills but they are a wonderful addition to your grill at any age. There are so many things to cook on a griddle that you may not have thought of. Obviously bacon and eggs, hash browns, French Toast, pancakes etc. But what about asparagus, carrots, sliced potatoes or sliced onions. And don't forget shrimp. The days of finding giant prawns that are too big to fall through the grill are over unless you live on the coast. You can also toast buns without turning them black, even do a grilled cheese. Some of the newer type burgers (not meat) can fall apart quite easily on your grill grates. Delicate fish can be a problem for a grill grate but not for a griddle.
There are 2 potential negatives to a griddle that you need to be aware of.
If you give up one of your grates to permanently mount the griddle, you will not have as big and are for cooking things like steaks. I seldom need grates on both sides of the grill but when I do, I flip the griddle to the grill side and even get grill marks (small ones). Worst case, you have two grill grates and swap one out when you need the griddle.
The second negative is rust. If you leave the grill unattended too long during your rainy season, you may get a surprise. Truth is, cast iron griddles need to be seasoned every so often depending on weather and usage. Many come pre-seasoned but you will need to do it again. I've included a video about seasoning on this site so you can see this is not such a daunting task. It certainly should not prevent you from using a cast iron griddle. There are many more seasoning videos on Youtube because cast iron pans require it as well.
I selected these 5 griddles base on their reviews - both the number rating and the owner's comments. The #1 is personal bias because it is the one I have and I can personally recommend it without hesitation!