In LTspice you can create a symbol automatically in two situations; either you have a subcircuit definition in ASCII format at hand or you want to create a hierarchy with your own schematics which are building blocks for larger designs. Hierarchical schematic drafting in particular offers powerful advantages by enabling larger circuits that can be drafted in a single schematic while retaining the clarity of the smaller schematics. Additionally, repeated circuitry can be easily handled in an abstract manner, i.e. black boxes, with full functionality and can be re-used across several schematics.
