"There are different types of shrub roses, some of which are able to resist disease and others which suit diverse climates and soils, features that are of benefit to both horticulturalists and gardeners"
I don't understand why in this passage they don't write "that are beneficial" instead of "of benefit". When we use "of benefit" and what's the difference?
"Of benefit" is useful when you want to use the degree "of some benefit". It's difficult to put across the same thing using "beneficial". You can try "slightly beneficial" but that might be too low a degree, "quite beneficial" but people's understanding of "quite" can differ widely, or "fairly beneficial" but again, it sounds unnatural. While "of some benefit" doesn't tell you exactly how beneficial it is, it's a useful phrase.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.