You are right, fertilisers are taken down into the soil by water, but there is a reason why you put fertiliser in the planting hole. Whenever I plant permanent items such as shrubs and herbaceous perennials, I always put the fertiliser I'm using in the planting hole, usually mixed with the soil at the bottom, and add some to the back fill soil I'll be replacing in a minute. I may also be mixing in humus rich organic material with the back fill soil too, though more usually this has been added when prior digging has taken place.
There's a good reason why this is done; nutrients in the fertiliser are taken up by roots, once they're wet - a granular fertiliser such as Growmore takes 6 weeks to break down completely, which means the plant's roots can access the nutrients more readily whilst they're spreading out and down into the soil, without waiting for fertiliser to percolate down from above. Placing fertiliser in the planting hole is the quickest way to allow your new planting to access nutrients whilst they get established, and it's the only time you'll be able to do that. Future applications of fertiliser, naturally, will be forked or raked into the topsoil around plants, and plants are then reliant on its percolating into the ground to reach the roots.
The exception to this is planting in autumn/fall, when you don't really want to encourage plants to grow too rapidly by adding chemical fertilisers, to avoid the risk of sappy new growth which won't have time to harden off before winter sets in.
As for planting annuals, that's the one time I don't worry too much about adding fertiliser in the planting hole - most annuals are rabid growers anyway, a lot prefer poorer soil, and summer bedding type planting will be receiving a higher potassium feed than shrubs and other planting, often in soluble form.
UPDATE: I watched the guava planting video with interest, I'd heard of this practice but never seen it being done. He's put a fish in the bottom, presumably some that's been in the freezer too long or is out of date, because it will rot down quickly, and in the process provide a fairly quick shot of organic fertiliser to the roots, in the same way that chemical fertilisers would. Better than inorganic fertiliser, because it is humus material, which increases biodiversity in the soil, which in turn makes your plants better able to grow.