Why ASP.NET routing add id into the url? - asp.net

Question

I am making menu with categories, where first menu item is All categories. All items call method Index in controller Auction, however, first item does not send id as a parameter. I am curious, why I can not call method from view without asp-route-id and I have to set asp-route-id to null for the first item? Otherwise, it will set id the id of last clicked menu item. Are there another way how to solve the issue?
Works:
<div class="menu">
#if (Model.CurrentCategory == null)
{
<a class="d-block active" asp-controller="Auction" asp-action="Index" asp-route-id=null><span></span>All categories</a>
}
else
{
<a class="d-block" asp-controller="Auction" asp-action="Index" asp-route-id=null><span></span>All categories</a>
}
#foreach (var category in Model.Categories)
{
#if (#category.Id_category == Model.CurrentCategory)
{
<a class="d-block active" asp-controller="Auction" asp-action="Index" asp-route-id="#category.Id_category"><span></span>#category.Name</a>
}
else
{
<a class="d-block" asp-controller="Auction" asp-action="Index" asp-route-id="#category.Id_category"><span></span>#category.Name</a>
}
}
</div>
Does not work (copy id from last clicked menu item):
<div class="menu">
#if (Model.CurrentCategory == null)
{
<a class="d-block active" asp-controller="Auction" asp-action="Index"><span></span>All categories</a>
}
else
{
<a class="d-block" asp-controller="Auction" asp-action="Index" ><span></span>All categories</a>
}
#foreach (var category in Model.Categories)
{
#if (#category.Id_category == Model.CurrentCategory)
{
<a class="d-block active" asp-controller="Auction" asp-action="Index" asp-route-id="#category.Id_category"><span></span>#category.Name</a>
}
else
{
<a class="d-block" asp-controller="Auction" asp-action="Index" asp-route-id="#category.Id_category"><span></span>#category.Name</a>
}
}
</div>

Answer 1

ASPNet MVC add Id at the end of your route becuase it is set as default URL Route pattern, passing the Id is optional. It is set up when you create the application. Have a look in StartUp.cs
routes.MapRoute(
name: "Default",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);
or Program.cs in ASPNet Core
app.MapControllerRoute(
name: "default",
pattern: "{controller=Home}/{action=Index}/{id?}");