It's not necessary, just very advisable to do it.
The lower the concentration is, the harder it gets to extract something. Ideally CO_2 is captured where it is concentrated the most, i.e. right after generating it.
Most of the concentrated sources of CO2 are used to produce energy. The equation is like
something + O2 --> CO2 + energy
We want energy. The problem is that we want energy.
If you try to reverse it with
CO2 + more energy --> something + O2
the problem is that you need more energy to reverse the reaction. If you stick one 100% carbon capture magical device to a power plant, you will not produce any energy in the power plant. Moreover you will need to buy a lot of energy.
There are some process that emit a lot of CO2, but use it mostly for the chemical reactions and not to extract energy like in cement production [1] and somewhat in blast furnaces [2]. You can capture the CO2 produced by them to get guilty free cement or steel, but is much much much more efficient to use the energy to replace the energy of a power plant that burns coal / oil / whatever. (Call it carbon emission trading.)
Once we close all the power plants that burn coal / oil / whatever, it can be useful to add carbon capture.
