Why some Collection methods return a new Collection instance rather than modify self property?

[liquid207]

Such as \Illuminate\Support\Collection::flatten()

    public function flatten($depth = INF)
    {
        return new static(Arr::flatten($this->items, $depth));
    }

This will lead to a problem:

User::all(); // result is `Illuminate\Database\Eloquent\Collection`
User::all()->groupBy('id')->flatten(); // result is `\Illuminate\Support\Collection` 

In the chain call, the data type has changed. What is the reason for doing this？

[MeesterDev]

FYI the cause of this is not the flatten() implementation you've added, but rather the one of the Eloquent Collection, which calls toBase(). I am curious what the reasoning for this is.

Seems it was added in commit 91a31e9

[liquid207]

This is also the point I am curious about.