To understand interacting particles, defining the theory on graphs built from world-lines and interaction vertices is required. This involves specifying data like coupling constants for each interaction vertices, which is cumbersome.
Instead, we use QFT on D-dimensional spacetime , where particles are regarded as excitations of quantum fields and interactions can be dealt with more efficiently.
However, in string theory, the situation is much better since the vertices are replaced by smooth world-sheets.
These interactions are automatic and the formulation is powerful (2-d Conformal Field Theory).