why use trap or mine?

Mine builds are pretty bad this league due to delirium just not being very friendly for it... but in other leagues, especially ones like metamorph and blight, mines were one of the most popular builds, here's some reasons why:

1: They can be extremely cheap to gear and still get good DPS
2. Single target burst damage is INSANE for any phase-type bosses - you lay out 20 or so traps, grief the boss, and then detonate them all at once: this method will still insta-phase Sirus AL8 fairly consistently
3. A ton of ways to add damage to your character -- spell damage, elemental damage, mine damage, etc. all add into your mine damage, making it easier to gear and easier to stack damage
4. Tremor Rod is BiS for a lot of builds and is A) Dirt cheap B) Somewhat SSF friendly and C) Allows you to use Kaom's Heart for extra life
5. Most mine builds have fast clear speed, insanely high single target DPS, and anywhere from shit poor to great survivability (depending on how the character is spec'd -- my AL8 sirus killer has 5.5EHP, but that's all it needs since it instant phases him for the most part).
6. For clearing you can literally just hold down throw mine and detonate mine at the same time, and then just dash through the map while your mines kill everything ahead of you
7. The passive tree/saboteur ascendancy offers a lot of eelemental penetration -- this is a HUGE damage buff that self casting wont get you
8. With self cast you typically only cast 1 or 2 times per cast (spell echo, helm enchants), with mines, you throw them all out and it casts multiple times while you maneuver around -- very fast cast speed compared to other skills.
9. Play style: Some people like it

Before Herald stacking and to an extent 3.9s zombies, mines were considered to be the OP build for awhile. 3.9 i'd say zombies took over a bit, but mines were still extremely popular. This league, the mechanic is simply not friendly for miners at all so they're not being played. But, other than Herald stacking I think mines probably have the highest damage output in the game.
For example: my current mine build is ball lightning. A burst of damage against a boss for me is 20 mines. My mines do approximately 400k dmg per hit (ala The Eternity Shroud) and I throw out 2x ball lightning per mine. Ball lightning also hits a target MULTIPLE times when it passes through them, for this example we will be conservative and say 3: That means that per sirus encounter where it's VERY easy to setup mines before hand i'm doing 400k damage * 2 balls per mine * 20 mines * 3 hits per ball => 48 million shaper DPS in one burst. Even if those are under super optimal circumstances, we can still easily say that its AT LEAST 20 million per burst. Plus, I can keep shooting more and more mines out as my other ones continue to explode. Further, PoB doesn't count the fact that eternity shroud will ignore Sirus' chaos resistance since I'm fully Shaper geared, meaning I have an extra 15% or so damage that i'm not counting (~1/2 of my damage is chaos, I believe AL8 is 30% chaos resist). Further more, if those numbers weren't enough... High impact mines gives us 2% chance per mine placed to do double damage, 20 mines * 2% = 40% for our mines to do double damage under ideal conditions. Basically... Mines can do a shitload of damage when you stack them on top of each other and burst them all at once. Since the wording implies that the % chance for double damage will be lower with each mine detonated, 0% with 1 mine and 40% with 20, we can average that to 20% chance to do double damage. So now we take 48 million + (96 million * .2) = 67.2 million shaper damage for a single mine burst under ideal situations (and since we're doing a single burst here, having Vaal Righteous fire and all flasks up is actually very easy to do). And then there's elemental penetration that PoB doesn't calculate correctly/account for, eternity shroud mitigating enemy chaos resist, etc. that add more damage.

TK;DR: Mines = big damage, and insane single target burst damage. Instant phases AL8.

lost of really good answers, ty
might try mines next league, depending on mechanic then