Manual routing allows you to select specific routing resources for your nets. This gives
you complete control over the routing paths that a signal is going to take. Manual
routing does not invoke route_design. Routes are directly updated in
the route database.
You might want to use manual routing when you want to precisely control the delay for a net. For example, assume a source synchronous interface, in which you want to minimize routing delay variation to the capture registers in the device. Assign LOC and BEL constraints to registers and I/Os, then manually route nets to control routing delay from the IOB to the register.
Manual routing requires detailed knowledge of the device interconnect architecture. It is best used for a limited number of signals and for short connections.