It can get confusing at the store when you look at the wide variety of putty knives and scrapers. Even just in the Richard's line of tools there is enough choice to make you wonder. First notice that some are rigid and some are flexible and some more flexible than others.
If you want to apply something, like window putty or even plaster, you will want a bit of flexibility. In fact the best window putty knife is extremely flexible. That allows you to squeeze in the putty more smoothly. Kneed the putty until it is soft and consistent. Hold a ball of it at the base of your fingers, near the palm of your hand. Feed it under the bent, very flexible putty knife and simply move down the window. You can easily pick up the excess on either side of the nice putty bead. Run your finger lightly back over the work to seal it to both the window frame and the glass. Technically, this is really the only "putty knife" of the lot -- the others are actually spatulas, scrapers and knives
Scrapers are used to take things off, and you want them to be rigid. In fact, if you are actually shaving wood or paint finish, you will want it solid and sharp. If you are just sliding under wallpaper, you want it quite thin -- less rigid but not actually flexible. The 9 in 1 tool is very useful for preparing a wall to paint. The hole will allow you to pull nails without a hammer. The point will scrape into the grooves of molding. The front edge will remove plaster bumps or wall paper. The inside curve scrapes excess paint off of your roller, and the list goes on.
When it comes time to drywall, you will need at least three different widths of spatulas - properly called "knives" or "drywall compound knives". The narrowest, about two and a half inches wide, applies the first coat of joint compound into the valley created by the indentation of the long edges of both sheets of drywall and beds in the tape. The 4 inch knife spreads the second coat out beyond the edges of the first coat. The 6 inch knife spreads the third and final coat out beyond the valley itself. If you are covering a joint where there is no valley, you will need at least a foot-wide knife or trowel. Some pros spread the compound out over more than a foot on either side of a butt joint, where there is no valley and there is a bump to hide.