X mirror editing support only transform operations, no creating additional geometry as well?

I thought this X mirror is a general symetry editing mode like Maya has. But it only seems to miror on the other side basic transform operations like move and scale, rotate. Cutting new edges and all th other stuff doesnt seem to… “cut” it . Also why its only on X ?

I know there is Mirror modifier but I dont want to cut and preepare my model everytime. Plus sculpting mode mirror seems to support all axes so wtf ?

X-Mirror isn’t intended to mirror creation of new geometry (which is cutting etc). Mirror modifier is much more convenient and powerful tool to use for anything to be mirrorred. Another option is Symmetrize which doesn’t require deleting half of mesh to proceed.

How is sculpting related here? Mirrorring for sculpting should be done with options in the brush settings, like Symmetry/Lock in 2.79.

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Using the modifier its not convenient all the time and also requires more steps. For example when you want to have only some details mirrored on the other side and not the whole thing. Symetrizing the mesh also requires additional steps like doing your editing , making sure to select the proper geometry to be mirrored, apply it and choose the direction. When you have more complex geometry the selection is not that simple and fast. So at this point I suppose this is an old undeveloped and of little use feature asking “what is my reason to exist”.

Having a real live editing symetry across all the axis, both object and global, alowing partial symetry that can be turned off and on to stop or continue working on your mesh in symetry mode is what Blender would need.

How is that topolgy mirror option any different I might ask ?

Separate it into another object and mirror as you wish. There’s also Interactive mirror feature which can be used on selection, no need to separate then.
Modifier requires additional step only once. Then it’s working and mirrors on fly.

In short, Blender by default doesn’t have live mirror tool without adding modifier and without using any operators additionally, whether it would be good to have or not. You can take a look at addons though huge amount of them uses modifier under the hood (still it might seem more convenient). Your question might receive less general answer if you provide certain situation with screenshots and an example file.

For someone it might seem asking that, for others it will not.

From what?

how is that topology mirror different than x mirror, as a concept technically and as usefulness?

See manual:

When using the X Mirror option to work on mirrored Mesh Geometry, the vertices that are mirrored must be perfectly placed. If they are not exactly positioned in their mirror locations then X Mirror will not treat those vertices as mirrored.
Topology Mirror tries to address this problem by determining which vertices are mirrored vertices not only by using their positions but also by looking at how those vertices are related to others in the Mesh Geometry

On example with monkey with Topology Mirror vertices on one side will be moved to place corresponding to one of their counterpart if they weren’t already.

that is like the begining of an useful partial symetry editing. These two combined with supposrt for geometry creation will allow a more flexible and in some cases faster symetrical modeling.

anyway thnaks for your attention