I also need this for creating a Mileage Reimbursement form that allows users to enter a month's worth of trips. Currently we use another solution but wanted to migrate it to Smart Sheet. The other solution uses arrays so you can fill out the trip information, then add a new row for each trip until the end then Submit. It also does the calculations for the entire form, which then is automatically sent to Accounts Payable.
The same thing for Petty Cash, with rows for each expense item.