Instead of doing all that work every frame, you do it once (at initialisation), and then simply rebind the appropriate VAO for each (set of) draw call(s) that use the associated vertex attributes.
Vertex Array Objects are used in addition to VBOs in order to improve client-side (CPU-side) performance, by reducing the number of calls needed to rebind individual vertex buffers and re-set vertex attributes every time you want to change to render in a certain way. Vertex Buffer Objects (which really aren't dissimilar from other types of Buffer Objects, for example Uniform Buffer Objects) provide a means for uploading vertex data to the graphics hardware. However I'm going to assume the OP's question should have been titled "when to use VAOs and when to use VBOs".) (I'm adding this here due to the the fact that ChrisE's answer is highly ambiguous, which is unfortunately due to the ambiguity of the original question. These are the only way to store geometry data in OpenGL ES 2 (and so, WebGL). OpenGL ES supports them (OpenGL ES 2 does not). These are available in OpenGL prior to 3.0, deprecated in 3.0, and gone in 3.1+. If you can get away with never reading the data on the host device (so, just pushing it out onto the card), VBOs in write-only mode are a good option. Vertex arrays are good for data that changes frequently but that also is read by the host machine-so, for directly rendering data that is being manipulated (laser rangefinder data buffers, for example, are where I've used them) frequently. VBOs are really good for static geometry like terrain that you don't expect to change, or for instanced geometry. Depending on the type of VBO created, you can give the graphics driver hints on the usage (write-many, read-many vs. The reason we prefer VBOs is that the data is loaded onto the card, and so you don't have to transfer it every frame. Here here is another good overview of performance issues in it we see that VBOs are more performant than arrays. Here is a good overview of the calling semantics.
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