I guess U can easily answer yerself if u think of what ever synths u have, now imagem that instead of osc.s U have samples... Also in the past the great magic of a sampler was the possibility of "virtually" have a certain sound without having 2 buy the respective equipment...
sample a few notes of a Piano and u have a "piano".. or sort of...
when it comes especifically 2 techno or ambient it's all about the possibility of using "sounds" (instruments, noises, vocals, efx, etc.. The sampler has made us much more sound aware.. u know that feeling of walking down the street hearing a noise and thinking.. damn' I could make a great track with that.. ;) )... reshaping, synthesis,etc...
Z