Why did they use mines in the 1st world war?

Answer 1

At Sea: To restrict & hamper movement of opponents' ships in strategically significant areas - eg the North Sea; the Channel; entrances to ports & naval bases.

On Land: Contact/ pressure mines were not widely used, but both British & Germans undertook extensive "mining work" (often employing battalions of miners drafted for this purpose) on the Western Front. Often this was to construct massive bomb & shell proof underground shelters (eg the British shelters at Arras; capacity >5,000!) in which assault troops could assemble prior to attacks.

Regular attempts were also made to "underpass" the killing zone of No-Man's Land: tunnels were constructed enabling troops to establish listening posts & sniper positions near the enemy front line. Trench raids were also launched from such tunnels.

There was a massive "underground war" fought out on the Western Front: each side tried constantly to tap into enemy telephone lines buried between front line trenches & headquarters to the rear; each would attempt to break into opponents' tunnels, & when this did happen nasty, brutal little close quarter battles took place underground.

Finally, but not least, the British, in particular, often started large scale attacks by detonating huge explosive charges ("mines") that had been pre-positioned in tunnels under the German front line. The mine exploded at Messines Ridge was so huge it literally blew a section of the ridge away & left a crater over 100 yards wide! The explosion was heard in London.