RedPlato

I use to play in PvP with perfect comps and I can say one can tell the difference when one's facing a team that is also maxed on IVs and when one's not. They take hits significantly better and also hit harder. I'd suggest to go 2x31 and 3x30 natured. A perfect comp can have 3x31 and 2x30 and be still maxed out, as 30 and 31 don't make a difference at level 50 if they don't have EV investment in those stats.
 
That been said, if you are breeding an offensive mon like Gengar, usually 2x31 (Speed and Sp. Atk.) and 27-29 in the defensive stats can also be fine.
 
However, with defensive mons, as said above by @ThinkNicer, maxed IVs become more important - I can't count the number of times my Reuniclus has survived at 1 HP, just to mention 1 example-.

Eggplant has answered a lot of it, but I made a guide video about how IVs and EVs work in competitive. I think it has every answers you need :D 
 

As better as your IVs get, the more chance you are giving yourself of winning. Simple as that. 
 
Also keep in mind some things such as:
 
- Physical moveset only pokes don't need 31 spatk (since they won't be using special moves) 
- Special moveset only pokes don't need 31 attack (in fact, they will want it as low as possible to minimize foul play/confusion damage) 
- Mixed moveset pokes will want both. 
- Trick room abusers / gyro ball users will want 0/1 speed IV
 
Another thing is, since we play lv50, you don't need 5x31 IVs when you are only investing EVs in 3 of them, the other 2 can be 30 IV and give the same stat as a 31.
Example: 
31/31/31/x/31/31 Blaziken with 4 Evs in HP, 252 attack, 252 speed
is the same as
31/31/30/x/30/31 Blaziken with 4 Evs in HP, 252 attack, 252 speed
 
The second option will be cheaper to make and gives the same value in battle as the first option. 
If somehow you find yourself wanting to invest EVs in 5 stats, then ye you need 5x31, but that's gonna be really rare occasions. 

thank you guys! Apologize for my ignorance about the old topic... anyway, I guess that my suggestions are not tauros**t! 

Your example is a bit bad as even -speed natured zard outspeeds conkel. 

https://calc.pokemonshowdown.com/ this can probably help you understand a bit better speed tiers and everything else. 
 

if you use the damage calculator that razimove used above, you can kinda see.
here's a comparison.
so having 31 IVs gives you about +15 stat points (at level 50).
having 252 evs gives you about +31 stat points (at level 50).
and having good nature gives you +10% (about +15 in this case).
 
as you can see, the evs are the most important.
but ivs and nature are not negligible either.
in total, a really bad charizard reaches 105 speed, but with all the optimization reaches 167.
 
so yes, it makes a pretty significant difference compared to a pokemon with complete trash stats, and all the optimizations quickly add up.
but like if your IVs are 29 across the board, and you have good nature and 252 EVs, you're only missing out on like +1 stat point.
 
it's usually not the end of the world if you do 1 or 2 less damage than you should. but speed literally determines whether you move first or second, so, every single stat in speed matters immensely when it comes to guaranteeing you are faster than the opponent.

People often say IVs don't matter much when you're starting out with PvP, but I'd say it depends on your playstyle. If you like more defensive teams, than IVs will matter more. Offensive non-mix mons are the cheapest to get into. Because you only really need 31 in speed. And even their attacking stat does not necessarily need to be 31. But you probably want something really close like 29.
 
Defensive pokemon are different because youre actually defending with two stats. Hitpoints and defense/sp.defense. Furthermore with most walls you still want to have 31 speed (if someone tells you otherwise they are bad). Because against a match up against another defensive team, that speed can be crucial. So you can see how defensive pokemon often need 3 or 4 very good IVs. 

IVs and EV investment (also nature bonus) come in to play whenever you battle, some conkeldurr might be running spdef investment EVs