>>104194381. They're cheaper then the fully painted ones. $10-$20 vs. $20-$30
2. Great for part swapping and building. If you have parts from 2-3 different figures plus joints you can easily end up with 4-5 different colors in a build. It will look detailed but decent compared to being overcluttered if all of the figure's parts were painted. A lot of figures come with alt parts, so getting a cheap blank will let you get almost a fully painted figure made out of your extra pieces.
3. This Bio-Masters drop was limited to just one of each in the first hour. By getting the blanks you can kinda get around the limit a bit.
4. If you like painting or modifying the figures, it's better to start with a blank. Both in terms of price and for the fact that you can't have to strip or prime the factory paint away.
5. Sometimes the base plastic just looks nice in a single color. Sort of a callback to bucket of army men, dinos, Keshi type toys, and the like.