WORKS AS EXPECTED:

{exp:switchee variable = "{segment_2}" parse="inward"}
    {case value="suchergebnisse"}
        <input class="form-control" name="keywords" id="keywords" value="{exp:low_googlesearch:keywords keywords='{segment_3}'}" type="text">
    {/case}
    {case default="yes"}
    <input class="form-control" name="keywords" id="keywords" placeholder="Suche nach z.B. Effi Briest, Arbeitsblatt …" type="text">
    {/case}
{/exp:switchee}

FILLS IN RANDOM (?) STRING.

{exp:switchee variable = "{segment_2}" parse="inward"}
    {case value="suchergebnisse"}
        <input class="form-control" name="keywords" id="keywords" value="{exp:low_googlesearch:keywords keywords='{segment_3}'}" type="text">
    {/case}
    {case default="yes"}
        <input class="form-control" name="keywords" id="keywords" placeholder="Suche nach z.B. Hebelgesetz, Perspektive …" type="text">
    {/case}
{/exp:switchee}