Signs alternate unpredictably, so the sheet trains reading the operation as much as computing it — the step most single-operation practice skips.
A child who has done twenty additions in a row stops reading the sign — and then meets a minus. Mixed sheets close that gap: each problem draws its operation independently, so attention stays on the symbol, not the pattern.
This is also the shape mental-arithmetic programs drill most, because running sums that rise and fall are what real calculation looks like.