The Midijob (officially the "transition zone") applies to earnings between €603.01 and €2,000 per month. The key advantage: you pay significantly lower social security contributions than in a normal job, yet still earn full pension entitlements. That's the big advantage over the Minijob!
Specifically at €1,200 earnings: normally around €240 in social contributions would be due (20%). In a Midijob, you only pay around €120-140 (10-12%). That's €100 more net per month! And best of all: your pension entitlements are calculated as if you had paid the full contributions.
Who benefits from a Midijob? Ideal for part-time workers, career starters, returners, or anyone who deliberately wants to work less. The Midijob makes part-time work more financially attractive, because social contributions don't rise proportionally. The full contribution rate only kicks in from €2,000 gross. Wage tax, by the way, is calculated completely normally based on your tax class – there's no bonus here.