As a way of demonstrating why players initiating rolls is a bad strategy.
Let's say I'm playing in a game where the DM does allow players to initiate their own rolls, and I find a cool shiny sword. Let's also assume, as the above example does, that this DM allows Arcana checks to be used to tell if something is magical or not.
Now, if I want to figure out if the sword is magical, I could say, "I make an Arcana check to see if it's magical," I open the door for that roll to turn out poorly, and for the DM to say "You can't tell" based on that result. But if I instead say, "I study the sword carefully, looking for runes, markings, or other signs that someone may have placed an enchantment on it," I don't open that door. Of course, the DM might say "Make an Arcana check to see if you find anything," at which point I'll make the check and abide by the results. But I'm also inviting the DM to decide "you know, that action you just described doesn't have a chance of failure. You see some runes engraved on the hilt that say 'Flametongue' in Draconic." Waiting for the DM to ask for a roll, even if you're allowed to initiate rolls on your own, gives you more opportunities for automatic success.