Expenses is for tracking payments made by people in your group, and updating each person with what they owe to who. The smart part about Expenses, is something called Circular Debt Resolution, which cancels out all the unneeded payments between your group. Read more about it here.
What are Purchases and Repayments?
A purchase is is simply any outgoing cost or payment made by any member of the group. It creates debt between the person who made the payment and the rest of the group.You can split the amount that was paid between group members if more than one of you paid, like if you went halves with someone on a large purchase.
You can also split the shares of who needs to pay back the money. So you can exclude people who shouldn't be involved in the repayment. eg. Unconfirmed trip members.
A repayment is simply registering that one user has repaid another user some of their debt. You could hand over cash in person, via internet banking or however you like, this registers the repayment and will adjust the balances accordingly.