Blocks or sheets
I understand why so many, especially amateurs, choose to paint on a piece of paper in a block rather than paint on a stretched sheet of paper.
It can be difficult to stretch a piece of paper and it takes time, it must be prepared several hours before you paint. You must have materials, board and paper tape, a wide brush. It probably feels like a complicated and difficult process that also seems completely unnecessary.
There are blocks that are easy to take with you, it can start painting at once, no preparations and other hassle. And the painting becomes quite flat when it has dried.
There are two different types of blocks, they have slightly different uses. There are spiral-bound blocks and blocks that are glued only on one side, so that you can easily paint several pictures in direct succession. Those blocks are usually small, they are meant to be used outside, they are excellent sketch blocks, something to just put in your pocket when you go out.
The second type of block is glued around all edges. Each sheet is fixed and the painting must dry before you can remove it and start the next one. The idea with this is that the watercolor should be flat when it has dried, it works decently but not well. The painting is almost flat, but not completely. It is this type of block that is used the most.
If you buy paper in sheets, you can divide it into the desired size, if you stretch the paper, you get deeper and nicer colors and a painting that is completely smooth when it is finished. In addition, it feels good to have done something for real, not taken a shortcut. Because this is the traditional way, select a paper of the right size for the subject, soak the paper, stretch it on a board and let it dry before you start painting on it.
A calculation example: A block that is 18X26 cm (7X10 inches) by the brand Arches and that contains 20 sheets costs SEK 285 ($ 32) at I B Wahlstöm. For the same paper in a sheet that is 56X76 cm (22X30 inches), they charge SEK 67:50 ($ 7.5) if you buy a ten-pack. This means that the price per m² is:
Block: ca 305 kr/m²
Sheets: 158 kr/m²
This means that you pay half the price for the same paper in sheets as in blocks.
As you can read about here, a good quality watercolor paper should be prepared with water before it works well for watercolor paint. It is not possible to soak a paper in blocks, you can moisten the paper on the surface, but do not soak it. And honestly, who soaks a piece of paper in a block and lets it dry before painting it? Since it is not possible in a practical way to prepare the paper for watercolor paint, sheets may be preferable from that point of view.
All paper that gets wet swells, when a paper swells that is stretched on a board it buckles just a little, when a loose sheet of paper swells it buckles more, but when the same thing happens with a paper that is fixed at the edges (as in a block, or taped with masking tape on some surface) then the paper swells without being able to expand, Then it has only one way to expand: upwards. It becomes very bubbly since it has no other direction to expand than upwards. A loose sheet buckles, a piece of paper in a block buckles more, the best is a stretched paper.
When using blocks, you can paint several smaller paintings on the same paper or use the entire paper, but you cannot make larger paintings than your paper allows. If you use loose sheets, you can divide them gradually, it is easier to adjust the size after painting. Most of my students who use paper in sheets, immediately divide these into smaller sizes. Then you lose the opportunity to adjust the size to the need. If you have the ability to store entire sheets, it is better to divide the sheets as you are going to use them, that way you have the ability to use a paper in the correct format for a specific subject. If you get small pieces left over, you can use them for color tests and the like.
As you have probably noticed, I am not a fan of edge-glued blocks, spiral blocks are another thing, they are practical to take with you out and do not claim to be anything other than sketch blocks. But the edge gluing blocks claim to be something that works immediately, without preparation. This is not the case, you get worse colors, buckling paper when wet, not completely flat paper when it has dried at a much higher price.
There are only advantages to paper in sheets, they require a bit of preparation but it is partly a craft to paint watercolor, so it is surely OK to prepare the paper before you paint on it.