• The finance minister looks after finances. That is, he interfaces with the national bank, controls tax income, the public debts, and tells the other ministries that "no you won't get more this year".
  
• The economy minister controls economic policy. That is, he proposes market rule changes (and direct interventions into the economy), talks with the business and industrial and trade union associations, privatises state companies (and nationalises private ones).

(Now, since nowadays everything is economy, and everything is to be marketised, the economy ministry is more likely to have conflicts with the other ministries. The most high-profile of these was with the environment ministry throughout the Schröder I, II and Merkel I governments; above all the Wolfgang Clement - Jürgen Trittin war over the control of energy policy in the Schröder II government.)

As for consumer defence: this field was added at the time Rernate Künast of the Greens held the ministry. With her, the agriculture ministry was more a watchdog over agriculture than a lobbyist for them, but conflict of interest was indeed noted afterwards...

_{*Lunatic*, n.
One whose delusions are out of fashion.}

Back in the fifties and sixties it was different, and the BMWi had acquired a reputation and was able and willing to make use of it. As there is little sub-ordinated administration („Verwaltungsunterbau”), and, in a market economy, nobody to issue orders to, it is important to have a good and trustful relationship with the public. Erhardt knew this and worked hard for it; in his Frankfurt years he used to invite journalists to discuss/teach economics after office hours into the central station restaurant for a „Frankfurter Würstchen” and a beer.
This is all long gone. Helmut Schmidt, perhaps inspired by Giscard's  super minister status,  in any case in tune with his „Weltwirtschaftsführer” ambitions, captured the banking section („Geld und Kredit”) giving him, and his successors, television time at international events and a lot political influence. When Oskar Lafontaine became Superminister in 1998, the  political section („Grundsatzabteillung”) left for the finance ministry, too. What remains, is a torso. – And it is probably not a good idea to have the finance industry pertain to the finance ministry – a ministry should have a certain breadth, I presume.

The minister Rexroth,