Although it’s possible for space missions to communicate data with Earth, the process is anything but fast. Voyager 1, for example, takes about 19 hours to send a signal back to Earth, and that lag only increases as the spacecraft gets further away.  For truly long-term, deep space missions, the significant amount of time it takes to send a signal isn’t going to cut it. The spacecraft will need to adjust its own trajectory without relying on ground navigation. That’s where pulsars come in.