Not as much as you might think. In the UK we have 1 year and 51 weeks before an employee has the right to claim unfair dismissal.
It is relatively straightforward to dismiss someone who doesn’t perform, behave or fit into your business within this time period.
Probationary periods do have the more subtle effect of testing out behaviour and attitude, providing they are long enough.
We recommend six months as a good timeframe, as it is hard for someone to keep up pretence over six months.
Probationary periods can be more beneficial when you promote someone.
You can promote staff on a probationary or trial period, with the clear message that in the event they are unsuccessful they will move back to their previous role and level of pay.
Assess each case on its own merits, particularly if recruiting at a senior or professional level.