I remember a conversation with my father-in-law. We were discussing the transition of my freelancing work into an employment. We spoke about the salary and benefits in the contract. He asked, if I had included a surcharge for being on a permanent task. I asked him to explain. And here is the bottom line.
We (in this industry) are knowledge workers. This is a much overly used phrase. It boils down to that our value as an employee or freelancer depends on our knowledge, be it explicit or tacit. If we are hired for a certain position, we need to keep at it. There are several possibilities how we can do that. Going to conferences, taking a training, meet colleagues etc. This needs time. And sometimes money. All this expenditure has to be accounted for. By you or by your employer. He may support you with money or free time or he may not (which is common in Germany). If not the salary has to compensate for your personal investment. So why has he to do that?
Because his order to work on a certain task or field for a (supposedly) very long time distracts you from acquiring additional knowledge. If you would still be freelancing, hopping from gig to gig you would to some extent learn new things. And you would invest some of your time and / or money in getting new things to know. Since you are your own boss.
If you work on a defined focussed task for a longer time, you will be left behind on your fields of expertise. This once was the reason you were hired. After some years, your worth as a coworker will degrade. And this is the reason good employers care for your knowledge.