Let's go with Intelligence now, so we move away from just Strength.
Character has 17 Intelligence, so +3, and is proficient in Arcana for a +5 at lower levels. They are able to reduce a difficulty by one whole category because they have more than just basic knowledge. 20 becomes 15, 15 becomes 10, etc.
This is quite a difference from someone with 8 Intelligence and no Arcana proficiency. -1 each roll. That means a 10 is an 11, 15 is 16, 20 is 21 and therefore impossible. So, more advanced knowledge is completely impossible for the person to know while someone who is intelligent and proficient in Arcana still has a chance of knowing something very obscure.
Arcana, a knowledge skill, represents how much a character knows about magic. Because YOU the player don't have an entire listing of everything your character should know, the roll represents the probability that - out of the vast sea of knowledge on the subject - you know details about a particular something.
So, for example, knowledge about Sussur Bark and how to use it to craft antimagic weapons should be a DC 20 or even 25 roll because Lenore was a pioneer on the topic. Maybe, just maybe, some information on it was spread by Lenore in Baldur's Gate when she returned there... maybe. Or maybe she told someone who then spread word about the few details that they knew. So, the DM might make a player roll to see if they even know some random details about it. Who has a better chance of having heard said details? Gale, because he's a Wizard and presumably a sage of sorts who has studied a lot of things like Netherese magic. His +5 then allows him to even have a chance of making the roll.
On the flip side, my barbarian with -1 Arcana has absolutely no chance at all in knowing anything about the topic because not only is he low intelligent, because he just doesn't care to learn such things, but he also doesn't focus on Arcana at all because, again, he just doesn't care about such things.
That said, take a situation where the Arcana roll is 15. The barbarian now has a chance. It's a slim one at 16 or higher, but it's a chance that, at some point in his life, he ran across details about this thing that he PROBABLY doesn't know anything about, but he might. So, a high roll is appropriate because there's a CHANCE that he might actually know something about.
Gale, on the other hand, would need a 10 or higher. He has a much higher chance of knowing it because he's done a lot of studying. However, should he just automatically know something just because he is proficient in Arcana and has high intelligence? Why should he just automatically know everything that is of a certain threshold? Does a student astrophysicist know everything about astrophysics? Each student of astrophysics might know different things because some things stuck in their brains and others didn't.
You have to remember that ALL the characters are low level in EA. So they aren't experts. They're maybe smarter than the locals, or stronger, or whatever, but they aren't experts yet. That comes at later levels and higher proficiency bonuses. There's even an Expertise feat that one has to purchase/earn that then sets the novices apart from the masters. A rogue with Expertise in Thieves' Tools may get a +10 to their roll with Dex 18 and Double Proficiency +6, while a rogue without Expertise would only get +7. That means the expert has a chance, though slim, of picking insanely difficult locks of 30 or higher, while the non-expert can't.
The point is, the roll represents, again, the CHANCE that you do or don't know something within a certain field, so it is appropriate to roll whenever there is a chance of failure. That's the whole point of skill and ability checks.
