There isn't really guidance for it because it is parental choice :)
In theory, there is no need ever to change to a backless booster, because there are many high backed boosters on the market which will accomodate a child far past the 135cm minimum to travel without a car seat, lots will also accommodate children up to approx 150cm, which is usually the height that they can get a good seatbelt fit in all cars. So you can keep them in a high backed booster right up until you don't need one any more, and it is beneficial to do this as high backed boosters offer head protection in side impact crashes. They may also be more comfortable for longer journeys.
However, you might want to use a backless booster, either because you have a high backed booster which is smaller and doesn't expand up to the correct height for your child, or sometimes if you have lower ceilings in your car, it's impossible to adjust it any higher without it hitting the ceiling.
It can also be a child preference thing - if you have a child who is small for their age and feels embarrassed about needing a booster seat, a backless one might be less visible and they can be more agreeable to it.
Or it can be for portability/practicality/cost. Backless boosters are easier to transport around and cheap to buy and you can just keep a spare one in the boot in case of any unexpected passengers etc. There are also travel versions e.g. Bubble Bum, Trunki Boostapak, Mifold, which can fold up extra small for example.
If you are planning to take the back off the booster you have, check the manual to see if it's allowed to use it like that, not all of them are designed to be used that way, even if it came in two parts originally, that is often only for transportation and not because it converts into a backless type.
Backless booster is a lot better than nothing, basically the only function of (any) booster seat is to re-route the seatbelt across the child's hips rather than their tummy. This is done equally well by a backless or high backed type as long as the child is sitting upright and awake. The shoulder restraint doesn't make a huge amount of difference, especially for older children over about 7-8 years / 125cm tall, although a good quality high backed booster with a good rating in side impact protection does have a protective effect in shielding the head from side impacts in an accident, and (particularly for younger children) the support of the high back can be more comfortable and help them sit properly, as well as keeping the shoulder belt in position over their shoulder.