What is, and what use cases have the dot "." in helm charts?

In general, . in Helm templates has nothing to do with files or directories.

The Helm templating language uses Go's text/template system. There are a couple of different ways a period character can appear there.

First of all, . can be a character in a string:

{{- range tuple "config1.toml" "config2.toml" "config3.toml" }}
{{/*             ^^^^^^^^^^^^
       this is a literal string "config1.toml"             */}}
...
{{- end }}

Secondly, . can be a lookup operator. There aren't any solid examples in your question, but a typical use is looking up in values. If your values.yaml file has

root:
  key: value

then you can expand

{{ .Values.root.key }}

and the . before root and key navigates one level down in the dictionary structure.

The third use, and possibly the one that's confusing you, is that . on its own is a variable.

{{ . }}

You can do field lookups on it, and you have some examples of that: .Files is the same as index . "Files", and looks up the field "Files" on the object ..

You use . as a variable in several places:

{{- $files := .Files }}        {{/* Get "Files" from . */}}
{{ . }}                        {{/* Write . as a value */}}
{{ include "mychart.app" . }}  {{/* Pass . as the template parameter */}}

. is tricky in that it has somewhat contextual meaning:

• At the top level, Helm initializes . to an object with keys Files, Release, Values, and Chart.
• In a defined template, . is the parameter to the template. (So when you include or template, you need to pass . down as that parameter.)
• In a range loop, . is the current item being iterated on.
• In a with block, . is the matched item if it exists.

In particular, the interaction with range can be tricky. Let's look at a simplified version of your loop:

# {{ . }}
{{- range tuple "config1.toml" "config2.toml" "config3.toml" }}
- {{ . }}
{{- end }}

Outside the range loop, . is probably the top-level Helm object. But inside the range loop, . is the file name (each value from the tuple in turn). That's where you need to save values from . into local variables:

{{/* We're about to invalidate ., so save .Files into a variable. */}}
{{- $files := .Files }}

{{- range tuple "config1.toml" "config2.toml" "config3.toml" }}
{{/* This . is the filename from the "tuple" call */}}
{{ . }}: |-
  {{/* Call .Get, from the saved $files, passing the filename .
       as the parameter */}}
  {{ $files.Get . }}
{{- end }}