Adding fractions (or percentages) can come up in a variety of case types and consulting situations. An example might be a market share situation (e.g., Company A and Company B supply business customers in the widget retail market, in which 3,000 widgets are purchased by consumers each year. Company A supplies 4 customers which sell 300, 400, 500, and 600 widgets per year respectively, and Company B supplies one customer that sells 1,000 widgets per year; what portion of which customer accounts can or should Company B try to take from Company A in order to get to >50% market share?) or a breakeven situation (e.g., Fixed Costs are X and Variable Costs are Y; Demand will be Z if we price at $A and 1.5x Z if we price at $B. How should we price the product?).
There are a variety of case situations where manipulating fractions may come up. The best approach (e.g., converting to percentages, doing the math in your head, writing it out, etc.) will depend on your skillset and the situation. I would always advise interviewees to simplify their math and never try to do calculations in their heads which they may do incorrectly. Writing it out is rarely a bad move. However, practicing mental math is a great idea regardless!
Hope this helps.