Gravel and small rocks have a tendency to sink down into a mud bottom over time and just disappear. If you spread the gravel deep enough and over a large enough area it should last a long time. But if you are trying to build nests with gravel 5 gallon buckets full at a time, any amount you are going to get placed likely will be gone in a year or two. Unless your pond bottom is bedrock.
The containers simply allow you to make a bed with a modest amount of rock and it stay there longer. The fish really get at it when they make beds and likely will move some of the gravel completely out of the container.
Another option that works before the pond fills up is to put some landscape fabric down with the gravel on top. That would work better for a fairly large area rather than just a small spot. The fabric keeps the gravel on top of the pond bottom instead of sinking away into the mud and disappearing.
An interesting side note, in a recent
pond I built dedicated to raising redear sunfish I put some mixed size crushed limestone in pockets around the pond perimeter. Basically took the tractor loader and made a cut down into the bank forcing a "ledge" out into the water area. Then with the same loader dumped about a third cubic yard of multi-sized rock in this void. The dirt wall created by the loader cut keeps the rock from migrating down the incline as the fish "work" the gravel into beds. Drove around the pond a while back and noticed about 4 beds, likely made by the dozen adult size RES I had stocked this spring (along with 200 fingerlings). Not a single bed was in the areas I created but instead in the mud clay bank. And so it goes.
The fish have their own idea of where they want to spawn.
I created a spawning bed in my main pond also before it filled. First year the fish never used it. It grew great FA though. They made beds where they wanted to. But subsequently the fish have used the areas I created. Something about the rock weathering in? I have no clue. I just know the fish do their thing to suit themselves, not us.
If you are interested in using containers another one that I have not used but I think would work well and is economic and tough are the
plastic tubs that are used to hand mix quickcrete type concrete. You can find them in Lowes or Home Depot right near where the sacked concrete is sold. They come in two sizes.
