Conjugated chains, especially if delocalisation does indeed take place, tend to be flat or at least stiff.
This results in a tendency to adhere to each others. In addition, their stiffness also means that not enough conformational space open up upon solubilisation.
In order to enhance their solubility, or at least their processability^, flexible chains are linked to the main polymer chains.
The presence of the side flexible chains can result in
a more exothermic solubilisation process, the entity of this being dictated by the exact nature of the long substituent as well the specific solvent counterpart (enthalpy variation of the process);
and perhaps most important and surely general as solvent independent, an extra entropic term (entropy variation of the process).
The impact of the above on the free energy of solubilisation can combine and lead to a soluble material, or at least one that dissolves in aggregates small enough to allow processability from "solution".
The solubility of flat stacking molecules can be enhanced by the same "trick".
Finally, the entropic effect can lower the temperature of evaporation (vacuum usually nevertheless required) of organic materials for opto-electronics. So that at least a kind of processability is attained.
^For sake of clarity as OP used the words "chemical processability". There is no much chemistry involved, the meaning is that the material must be easily processed from melts, solutions, evaporation, etc. Literally "intractable" materials (no melting point nor glass transition, insoluble) were indeed the first conjugated polymers - willingly (polyacethylene) or not - prepared.
Another note specific for the discussed conjugated polymer. The particular branched chain was introduced to further reduce interchain interaction and so enhance fluorescence. In fact, one of the first envisioned application (nowadays mature) of conjugated polymers & molecules was electroluminescence for LEDs.