Although the question is to cabin crew, the seat belt sign is actioned by the Captain or First Officer under the command of the former.
It sometimes happens that a request is made to action the routine use of the sign slightly earlier than usual during a descent, in order to provide the necessary time to secure the cabin, which might be anticipated to be extended for one reason or another. Likewise in the cruise there may be extenuating circumstances that give rise to a request from the cabin crew to ensure passengers return to their seats and such a request would be considered as the occaission arose.
However the idea that it gives the crew "a rest" isn't true. When the sign goes on, there are interruptions in the cabin service and a requirement that the whole cabin is checked for compliance. The cabin then needs to be monitored for the duration of the required period. It is likely to be self defeating if the signs are continually used for no obvious reason, which is why there should normally be a brief explanation from the flight deck or the cabin crew if this occurs.
Although it is normal for the seat belt sign to be switched on when turbulence is encountered, it is also worth remembering that the flightdeck crew are constantly receiving reports from ATC and other aircraft concerning encountered conditions ahead. As well as this, they have preflight briefing material showing forecast areas of turbulence as well as (in many cases) the experience of perhaps covering the same reciprocal routing earlier that same day. Pre-emptive action is therefore common and desirable. Sometimes it turns out that the expected turbulence doesn't materialize in severity or indeed at all, however that is simply a bonus, and not a cause for not acting in an otherwise timely manner.
It also often happens that reported or likely episodes are deemed sufficient for ensuring the passengers are secure, but deemed sufficiently low risk to allow for continued cabin service. This is nearly always a judgement call, but it is made in the knowledge that with an otherwise secured passenger cabin, the active crew can complete the full security requirements in a much shorter time than would otherwise be the case if passengers were not already seated with their seatbelts fastened and checked.
In point of fact, the crew are busy doing their services and look forward to completing them, rather than having them interrupted. Certainly it can sometimes be frustrating when congregations of people impede a service, but experienced crew are normally good at sorting out those things. The seat belt sign is a safety device, and shouldn't in the ordinary course of events be used as a tool for anything else.