There are any number of reasons to add an external fuel pump. The obvious choice here is to replace a mechanical pump either to eliminate a bad pump or to accomodate an upgraded fuel system.
There are a wide variety of choices out there but the list is shorter when ease of installation and finding a pump are added into the mix. I will cover a few of options here. As a small side note it appears that there are only two primary manufacturers of aftermarket replacement fuel pumps here in the states, Walbro and Siemens. You will see them in a variety of labels and colors. The pumps that Ford uses are apparently made by Bosch.
Master E2000 / E2182 / NAPA TFP2P74028 / Carter P74028 This is the replacement unit for the external high pressure pump found on most 83-86 Fords with CFI. Also used on 86-91 Ford Aerostar, Bronco, Ranger, E150 to E350, F350/450. This pump is a good all around pump that can put out up to 95 PSI and 30/40 GPH. Pretty complete and easy to mount. E2000 has Ford quick disconnects and cost around $80.00 / E2182 has barbed fittings and cost around $75.00
Carter P5001 / NAPA P5001 / Holley HLY-12-927 This is a self priming pump. It regulates out 9-22psi.
Walbro FRA-1 This is a good low pressure pump for carborated applications. Capable of 20 GPH at 4.5 - 7.0 PSI and a low current draw of about 2 AMPS.
Master E8248 - Universal 60 gph and 125 psi max.
Master E8012S - Universal pump 6 PSI has inverted flare fitting on both ends. About $45.00.
Master E8228 is a high pressure fuel pump capable of 90 psi pressure that has been designed to be used in retrofit situations. It comes with a mounting bracket and 5/16" fuel line barbs.
Master E8094 is a medium pressure fuel pump made for the GM throttle bodies which run at 15 psi.