All-purpose means you can use it in any situation — one thing that does many jobs. All-purpose flour in cooking, an all-purpose cleaner, an all-purpose tool. It is a practical shop-and-kitchen word, not a grand one. It tells you the thing is not for one special job only. Though actually all is a promise on the label: an all-purpose knife will not replace every knife, but it will do well enough for most home tasks. In Taiwan, a US recipe that says all-purpose flour usually points you toward 中筋麵粉, the everyday bag, not a perfect copy of every American flour. It has nothing to do with on purpose (meaning you meant to do it).