Like postgoodism's answer states, it's Khronos api that basically does the same thing as WGL, GLX, and other platform-specific "gl bootstrapping" libraries do. It also provides some cross-api functionality, like using OpenCL with OpenGL while sharing some resources. In theory, at least. =)
- if a platform supports OpenGL ES 2.0+ or OpenGL 3.0+ i can presume that will support EGL too ?
No. Desktop OpenGL doesn't need EGL. Practically all mobile devices that support OpenGL ES are based on EGL, though (I haven't seen a single one where this isn't the case, but then again, I haven't played with iOS devices for example, so I may be wrong).
- when is convenient to use EGL and when it's not?
It's convenient when you're not using some higher-level bootstrapping library like SDL or GLFW, and EGL is available. On the other hand, platform-specific library example code is probably more easily available. YMMV.