There is a general trend, especially in Western Europe (it depends on the country, so putting a country tag might be good; your profile says you're in Estonia, and I'm not very familiar with the culture there) for employment to be indefinite; not continuing to employ someone requires significant cause. In some jobs, there is explicit tenure, but even in jobs without it, there's often "pseudo tenure" in the sense that laying someone off is a Big Deal. This makes hiring a new employee a large risk, as even if they have experience, it's hard to know whether they'll work out. Having a probation period allows a company to see how someone does in a position while giving the employee notice that the employment doesn't have this "pseudo tenure": the employee shouldn't consider employment after that period a sure thing. Thus, they can evaluate the employee at the end of the period and lay them off if they want without it being as much of a violation of social norms.
This also allows a company to terminate the employment with less of an effect on other employees: if the other employees are past their probation period, and they see an employee in their probation period being laid off, that doesn't make them worry about their job security as much as seeing someone with the same employment status as them being laid off does.
Another factor is that companies often have perks that they don't want to give to employees until the employees have established themselves. So there may be certain benefits that aren't available, or aren't fully available, until after your probation period.
The characterization "the employer is unsure about my qualities" is true in some sense, in that they are exercising caution about a new hire, but it's doesn't necessarily mean that they have concerns specific to you; this likely is simply general practice.