Being a presidential democracy, in Argentina the president has significantly more power than a prime minister or a chancellor in the EU and does not depend as much on the parliament.
Argentinian presidents often govern by decree bypassing the bicameral parliament (= senate & chamber of deputies). Macri had done it as he had no majorities in the parliament. But Albero Fernandez has pushed the envelope during his mandate with the DNU's (decreto de necesidad y urgencia). He passed decrees that went far beyond what decrees regulated in previous administrations. And that even after the parliament was functioning again (after the full lockdown). In fact, many experts argue that especially his decrees/DNU's issued after the parliament was functioning again were in part unconstitutional.
So, what will change after the midterms in terms of power of the government. Probably not too much, as Alberto will most likely continue to govern with decrees and not depend much on senate and chamber of deputies. But if they lose a lot of their seats in the parliament, it could show that the people who previously voted for them are very dissatisfied and that they urgently need to turn things around until the presidential elections in two years. Putting pressure on the government to fire some ministers and change some of their policies (or just sell a lot of cheap smartphones and TV's to the masses).
And today Argentina is voting the primaries. In theory determining who will be the candidate for each party/coalition in November. But this is unfortunately not true for many parties, as many only give you one option today. Only some have multiple options for senators and deputies to choose from. In reality today's primary elections serve more of a forecast of what the November elections will look like.