This is clearly part of Maduro's response to the ongoing political and economic crisis in Venezuela. Beyond that, supporters and opponents of Maduro have different perspectives on what this is about.
According to Al-Jazeera, Maduro himself told a crowd of supporters:
"I convoke the original constituent power to achieve the peace needed
by the Republic, defeat the fascist coup, and let the sovereign people
impose peace, harmony and true national dialogue."
The themes of "peace" and "dialogue" seem to be a common one raised by Maduro supporters. One of his advisers is quoted in the New York Times:
“There are violent groups that don’t understand that violence won’t get us anywhere, and that we must have a dialogue despite our differences,” said Hermann Escarrá, a legal adviser to Mr. Maduro.
Opposition political parties will not have representation in the assembly, which will give Maduro considerable control over the outcome. The assembly has the power to call for an election and to set the terms of it. ABC News cites a political analyst on the strategic implications:
If the constitutional process goes forward, opposition leaders will
need to focus on getting at least some sympathetic figures included in
the citizens assembly. That could distract them from the drumbeat of
near daily street protests that they have managed to keep up for four
weeks, political analyst Luis Vicente Leon said.
"It's a way of calling elections that uses up energy but does not
carry risk, because it's not a universal, direct and secret vote,"
Leon said. "And it has the effect of pushing out the possibility of
elections this year and probably next year as well."
The New York Times article quotes a different expert with a similar opinion:
To many, Mr. Maduro’s call to rewrite the Constitution seemed like an
effort to divert attention from the political crisis he has been
facing in the streets and to avoid elections that his governing United
Socialist Party is likely to lose, according to polls.
“From my point of view, they are in a dire situation that they don’t
know how to continue using the normal mechanisms, such as having
elections,” said Enrique Sánchez Falcón, a legal expert and professor
at the Central University of Venezuela.