With one share you may have the right to vote. With one vote per share, that won't bring you very far.
More importantly: with one share (or a larger minimum, depending on policy) you may have the right to attend and speak at the meeting. You can try to convince others of issues you find important (of course they should be related to the business of the company involved). Exactly this strategy is followed by shareholder activists, which may be political activist groups for social or environmental change:
Some of the issues addressed by shareholder activists are for social change, requiring divestment from politically sensitive parts of the world, for example, greater support of workers' rights (sweatshops) and/or more accountability for environmental degradation.
Here is an article by Greenpeace calling on people to use shareholder activism to hold corporations to account:
And if you do have shares in fossil fuel companies, use your voice as a shareholder and be an activist in the boardroom! Greenpeace is working with shareholder activists all over the world to turn their voices into megaphones.
This is just an example; issues raised by shareholder activists can be many. You may need more than one share, but for larger political groups, investing €1000 (or whatever the minimum is) in Shell or Exxon in order to get the right to speak at their shareholder meeting is an entirely realistic prospect.
Of course, whether such shareholder activism is actually effective is a different question altogether, and beyond the scope of this question (and probably off-topic on Money SE).