Certain abilities require a free hand to perform: picking up an item or drawing a weapon, casting a spell with a somatic component, using Gloves of Missile Snaring, etc.
If you make ana two-handed attack with a Versatile weapon, for how long are both hands occupied? Only during the instant of the attack? Until the beginning of your next (one-handed) attack? Until you explicitly take a free action to change your grip during your turn?
Can you drop a weapon (or other object) as part of a reaction to instantly have a free hand? (For example, to use the aforementioned Gloves of Missile Snaring)
Can you change your grip on a versatile weapon as part of a reaction to instantly have a free hand?