For example lets say we use

[dependencies.serde]
version = "0.8.*"
optional = true
default-features = false

and we want to use the std feature of serde if it is used.

[features]
std = ["serde/std"]

But now if someone wants to use our std feature serde WILL be imported. If I would want to represent this I would write

[features]
std = ["serde", "serde/std"]

Right now it's impossible to use a feature of an optional dependency from an own feature without forcing it to be used.