My qustion falls in to parts:

1. Why should I use it? I have never used strongly typed models, but just used Model.Value("xxx"). But I guess there are reason to why models builder exist, i.e. that there are benefits from using it.
2. How should I use it? I have figured out how to use it, I think. It works, anyway, an renders the desired content.

Template PageHome.cshtml

@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<ContentModels.PageHome>
@using ContentModels = Umbraco.Cms.Web.Common.PublishedModels;
@{
    Layout = "Master.cshtml";

    var TestComp1 = Model.TestComp1?.ToString();
    <p>@TestComp1</p>
}

Template PageDefault.cshtml

@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<ContentModels.PageDefault>
@using ContentModels = Umbraco.Cms.Web.Common.PublishedModels;
@{
    Layout = "Master.cshtml";

    var TestComp1 = Model.TestComp1?.ToString();
    <p>@TestComp1</p>

    var TestComp2 = Model.TestComp2?.ToString();
    <p>@TestComp2</p>
}

Properties TestComp1 and TestComp2 comes from two separate compositions and TestComp1 is added to document type PageHome while both compositions have been added to PageDefault.

Then in template PageHome.cshtml I insert a partial view:

@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<PageHome>
@{
    var TestComp1 = Model.TestComp1?.ToString();
    <p>@TestComp1</p>
}

This works fine, but if I insert it intro template PageDefault I get an error:

ModelBindingException: Cannot bind source content type Umbraco.Cms.Web.Common.PublishedModels.PageDefault to model type Umbraco.Cms.Web.Common.PublishedModels.PageHome.

And it makes sense, for sure. But is there no way that a partial can be used in templates where the model isn't sthe same? I seems inflexible to me since I don that often in the "old" way without models builder.