Only reason to play HL1 is for the story...
Ah-hem,
You are a theoretical physicist who specializes in long words was part of an experiment that gone wrong, destroying the base and opening another dimension. Then, the government steps in and tries to kill everyone and keep things quiet. Two thousand dead zombies and aliens later, you're at the end. Where a man in the suit recruits you for another job (Hl2).
Hl2 takes place later in the future, where aliens (combine) rule the world (or at least City 17) and you have to kick sommoar ass.