Hello, I have a noob question. What is a point of using 2.5d graphic like this for example http://steamcommunity.com/games/427520/announcements/detail/818916001147049328

I have to create 3d model and then cut it to 2d or there is another option?

2.5d graphic like this

Unfortunately the term 2.5d has been abused by enough people that it often no longer means what it originally meant and is beginning to lose all meaning in the field.

Originally it was used for first-person-shooters that appeared to be 3D but were actually 2D maps with some visual tricks thrown in. Wolfenstein 3D (1992) and the original Doom (1993) both fit that description. Today people use the term for the original meaning, and for isometric views, and for constrained views, and for anything that looks realistically rendered, and for more meanings besides. This seems to be your case here.

What is a point of using 2.5d graphic like this

That specific game, Factorio, is a 2D game where the images are rendered and animated in a style that looks 3D. The game appears to be made up of rectangular tiles, and each tile has artwork.

The art style happens to look like 3D pipes with fluid rushing through it, or as towers and buildings sticking from the ground, or as units standing up, but they could have made any art style they wanted. Even so, they appear to be 2D images, basically the same as jpeg and gif and png files. (Games typically don't use those formats as there are more efficient graphics formats that can be transferred directly to the video card.)

As for the point of doing it, the way the game world works is often an important consideration. By keeping the gameplay world flat the design stays close to many table-top games and card games. The design is laying tiles down on top of the game board, either replacing or augmenting existing tiles.

They have kept the game world effectively as a flat map where players interact from above with tiles of objects. Design-wise this means many other choices fall into place as a result of the decision. Keeping the game as effectively a tile-based board game allows for several common picking-based user interfaces, and allows worlds to be manipulated as grids and tiles. It has design implications to game strategy, to character pathfinding, to space usage, and more. It also has non-design technical implications to how the world technology is built, how data files are manipulated, how rendering works, and so on.

I have to create 3d model and then cut it to 2d or there is another option?

If you are making the artwork you can use whatever you want. You may choose to render it as views from a 3D rendering program. You may choose to take photographs of real objects and edit them down as needed. You may choose to use carefully drawn 2D art. As these appear to be basic tiles you can draw the images using whatever method you choose, in whatever art style you choose, as long as you ultimately get them into the game in the required formats.

In this case they chose to make an art style that looks like 3D with a nearly top-down perspective. Visual styles are important when building a brand and theme for a game. Some games incorporate multiple art styles and different tile or model sets, some games rely heavily on a specific look and feel for their brand. That was a choice they made, if you aren't part of the company you can do whatever you wish.

They could have chosen any art style, but that's the one they chose for whatever artistic reasons they had.