Both Jon & Tyrion already TOLD Dany exactly.... WHY... but apparently, it flew right over your head.
Skip to the 2:00 minute mark...
When Tyrion says: "Yes, you'll need to be ruthless if you are going to win the Iron Throne... You'll need to inspire a degree of FEAR.. but fear is all that Cersei HAS... it was all my father had... and Joffrey... but (fear) makes their power BRITTLE... because everyone beneath them wanted to see them DEAD."
- That's WHY Tyrion doesn't want her to cause MASS CHAOS & FEAR by unleashing her Dragons.

- He doesn't want the civilians of King's Landing to 'wish death' upon Khaleesi the 'day after' she sits on the throne.
It's not that hard to figure out... if you paid closer attention.
Now skip to the 0:17 second mark...
When Jon says: "I never thought that Dragons would exist again... No one did... The People who follow you KNOW that you made something impossible happen... Maybe that helps them BELIEVE that you can make OTHER impossible things happen... Build a world that's DIFFERENT from the shit one they've always known.... but if you use THEM [*looks at her Dragons*] to melt castles & burn cities... you're NOT different... you're just MORE OF THE SAME."
- That's WHY Jon Snow doesn't want her to cause MASS CHAOS & FEAR by unleashing her Dragons.

- He doesn't want her to be thought of the same way as her father... because the civilians of Kings Landing will 'wish death' upon Khaleesi the 'day after' she sits on the throne.
It's not that hard to figure out... if you paid closer attention.
No, I don't think you do.
Think about this, to get the everyone on his side to fight white wallkers, he abandoned Winterfell to solicit help from Dany and get Dragon glass.
Jon never
ABANDONED Winterfell.

He trusted in
SANSA to...
'hold the fort down'... until he RETURNS.
There's a big difference between just
'peeling off without a 2nd thought'... and
RELYING on your family members to
'take the wheel' (temporarily).
He gets Dragon glass but now he needs help from Cersei so he goes North of the Wall with a crew to attempt one of the dumbest fucking plans I have heard that is extremely high risk (what if dies) and low reward (Cersei could tell to fuck off, I have my own White Walker).
They need help from EVERYBODY.

All over Westeros. Where you been?

Practically everyone has
'a part to play' in the Great War to come.
But the ONLY WAY to
CONVINCE people like Cersei....and/or the ArchMaesters at the Citadel.... is to LITERALLY 'SHOW THEM'... that the Army of the Dead...
is REAL.
- Was it a dumb plan? Nope. Not really.

- Was it a dangerous plan? Absolutely.

- Was it an inadvisable plan? Extremely.

- Did it work? Yes. But there was a 'price' to pay.
Now if you got
'A MUCH BETTER PLAN' to convince...
the entire 7 kingdoms (very quickly)... that the Army of the Dead is REAL... then let's hear it.
Until then... sit back & enjoy the tv show.
Of course the real reason he went to the wall was so the writers could get Ice Dragons. This is shitty writing that never would have been in the books.
"The books are the books.... the show is the show" 
- George RR Martin
Obviously, after 7 seasons of watching the Army of the Dead
'just run amok' North of the Wall, killing all living souls... a single Ice Dragon is waaaay MORE IMPORTANT than getting all the 'living folks' in Westeros to help fight the Night King... in your opinion.
The whole purpose of the White Walkers... is to ride a Dragon. Not kill everyone.
Sure, I hear ya.
(Guess you never stopped to think that maybe the Ice Dragon could be used to knock The Wall down, somehow.

Because it's shitty writing.

I get it. They should have a MUCH more convoluted plan to bring the Wall down, right?

)