To the contrary belief of certain persons, seeds and seedlings have many ways of being useful to bonsai enthusiast. Ill elaborate on a few of them.
Seeds are of many uses and the first one them is to have a tree to develop into a bonsai plant. Pretty obvious, no? But other then growing a future bonsai tree, what could you do with tree seeds?
Another use for seeds is the potential to develop a new cultivar. Every seeds are a different gene pool then its parents. This is why; some say a cultivar can only be reproduce by cuttings. This is true. But from a seed can come an even better cultivar. I like to think of it as the lottery. I usually sow around thirty seeds of the desired species. Average germination rate of tree seeds is 75%. So 75% of 30 seeds is around 22. A month after sowing, I usually get 22 seedlings. From these 22, I will prick out the weakest and nurture the strongest. After a few weeks of growth im usually able to distinguish the fastest grower (which is very interesting) but im also able to find the ones that grow compact with short internodes. I then select the 10 best seedlings and bonsai plant candidate to be moved to their individual pot. I called this method discriminative growing. This leaves me with excellent bonsai stock, which I can then propagate via cuttings.
A third uses for seeds/seedlings, is to grow many thin seedlings that we will use for a graft later. You could either graft some roots to accentuate your nebari or even graft a branch. We all have looked at a bonsai tree and wished that a bud would break at a very specific point. Well you could do it with a graft from side to side.
I also have seen some people who will do a root over rock bonsai tree by using many tree seedlings that they wrap around a rock then join the seedlings together over the rock. Roots and seedlings will eventually fuse with the rock, but seedlings will eventually fuse together on top of the rock.
One other thing that I find seeds useful for is that they can be a source of uncommon species pretty cheaply and easily. If you don’t have access to a certain species of tree in your area, you could always buy some seeds of your desired species and grow them. Shipping tree’s can be tricky and cost a lot. Seeds are easily ship around the world.