A windshirt saves your shell from being used when its cool but dry. That means your shell will last longer. A windshirt is about 15%-35% the cost of a very breathable shell and its core function is not noticeably impacted by wear. On a long trip, when at the times it is actually raining sufficiently hard you start using your shell for what its good for having kept it from damage and dirt letting the windshirt take the abuse.
Over a 5-10 year period, the windshirt will pay for itself many times over.
A windshirt evens-out the feeling of temperature during a variable wind-speed situation which is common in hilly areas. Imagine turning a corner wind hits you and you're cold and 2 mins later out of the wind and then hot, etc. Means less bothered by the variable conditions.
A windshirt turns a fleece into a non-waterproof coat by preventing the wind from ripping through the fleece.
A windshirt saves you $ by allowing variable weight insulation to be bought to follow the seasons, and layer the one windshirt over.
A windshirt can be an emergency extra layer if your shell is damaged as it reduced how much water gets to whatever is the damage in the shell. Combined with my first point above means the windshirt extends a shell's usefulness.
A windshirt can afford a degree of water resistance, blunting, deflecting slowing and so if you don't actually have a waterproof, but a water-resistant something,it will extend how long you can handle before soaked+cold.
One of my most used pieces of kit is a windproof vest, quite tight on the torso as it doesn't need the room to allow for shoulder as you rotate arms so it add "just enough" insulation to the core. My next most common is a windshirt jacket with somewhat baggy sleeves to pull/roll up. I can combine these to then pinch the baggy jacket with the tight vest.
To also balance the pro-windshirt, it will not come with any insulation by itself, if you put it over bare skin as its thin fabric it will conduct cold air temperature, but over a long-sleeve zip-neck baselayer it turns the baselayer into a fleece type insulation when no wind.
I much prefer the zipped jacket type. I have hooded one but thehood is barely used, if I were buying from scratch I'd go for a hoodless jacket.
I visit UK about twice a year and my fave windshirt is Montane, usually about $50 or less.
I live/hike/bike in the northern California coastal area and temperatures and windspeeds vary about 40F through a few hours and I carry the windshirt in afternoon and wear it as wind picks up, I know its not going to rain for a few weeks/months. Whilst not waterproof its sufficient for me to walk or bike through cold damp fog too.