So, there are a couple of parts here, which I'll try to answer in a simplified and succinct manner. I've been to North Korea, have an interest in socialist history/politics, am familiar with the DPRK's constitution, and consume DPRK media/radio/tv and other publications somewhat often, so that's what informs this answer.

Who decided that there needs to be a kind of national assembly?

The current constitution was first approved by the Supreme People's Assembly in 1972, although the SPA has existed since August 1948, following the example of other socialist states. Constitutionally, the DPRK is a socialist state led by the Supreme People's Assembly. The constitution notes: "The sovereignty of the DPRK resides in the workers, peasants, working intellectuals and all other working people." Basically it is required for the country to have some form of legislature, in this case the Supreme People's Assembly, in order to let minority groups or different interest groups propose new policy.

Why does North Korea have a parliament? ... It seems like you could simply have "presidential elections" every few years and be done with it.

The DPRK is a socialist state and is not unlike other socialist states in structure. For example, the German Democratic Republic (East Germany) was also led by an alliance of multiple parties and political groups (the National Front of the German Democratic Republic), which would elect members to the legislature (Volkskammer). Each group could win seats in the Volkskammer and represent their own separate interests, but the Socialist Unity Party was the "vanguard" party which had the leading role in the Volkskammer.

The DPRK is not that different: the Supreme People's Assembly is made up of different parties and political groups which represent different interests. There are groups representing religious interests, social democratic interests, interests of the Korean diaspora, and sometimes some independent politicians. However, the Workers' Party of Korea is the vanguard party of the DPRK which leads the Supreme People's Assembly.

Socialist states are often criticized for having "show elections", but these types of governmental structures more or less correspond with a Marxist-Leninist (or more broadly socialist, as the Workers' Party of Korea is not a Marxist-Leninist party) conception of governmental democracy: there exists a vanguard (communist) party which will lead the state, and other groups will be allied with it. However, those other groups — whether they are labour groups, women's groups, religious groups, ethnic minority groups, etc. — are still encouraged to lobby for their own interests.

There is a principle pioneered by Leninist parties called "democratic centralism", which at a very basic level means that a vanguard party will organize sessions in which members can propose new ideas, and then debate and discuss them freely. Once a vote has been called and the majority decides a course of action to take, that action will be implemented. However, even if some individuals or groups disagree, after the decision has been made you have to act in a united fashion — meaning, you can't air your grievances with the new policy as the majority has made a decision. The WPK is the vanguard party, and the Supreme People's Assembly is the session in which new ideas can be proposed.

For example: I am a member of the Supreme People's Assembly and a session is called. I represent a labour group. I feel that, for the benefit of workers, the minimum wage should be raised to $20/hour. However, a different party thinks it should stay at $15/hour, and another party thinks it should be raised to $23/hour. We discuss the pros and cons, and after long discussions, the majority votes to keep the minimum wage at $15/hour. I'm probably unhappy with this decision, but I am a member of a united front and I recognize that the majority voted for the minimum wage to stay at $15/hour. Thus, I can't go out and start complaining on the news and in public that this was a bad decision, because that would betray the cooperation and united-ness of our group. However, the next time the Supreme People's Assembly is called, I can propose it again and we can vote on it another time, and maybe our decision will change.

The DPRK is of course led by the Workers' Party of Korea, there's no question about that — but any member of the Supreme People's Assembly can propose new policies. Kim Jong-un is the chair of the Supreme People's Assembly and the WPK, but technically doesn't have the final say: I'm sure his opinion sways the vote but there still has to be a majority decision. Whether or not these elections and votes are "show elections" is contested but the North Korean system is not that different from other socialist governments.

I hope this answer makes sense!

Edit: I should also supply some recommended readings/sources.

• Of course, the DPRK's constitution figures pretty heavily into this answer, so I'd recommend that first. This isn't the most recent version but is mostly consistent. Chapter 6 deals with the structure of the state and the Supreme People's Assembly. This is the 2017 version but you may not be able to access North Korean websites depending on where you live.

• The DPRK publishes documents from each Supreme People's Assembly through the Pyongyang Foreign Languages Publishing House. I would recommend taking a look at these (your local library may have a copy) to see the proceedings of each session. Here's the Google Books page for the 1960 publication. The Pyongyang Foreign Languages Publishing House also sometimes publishes individual speeches made at the SPA (like those by Kim Il-sung/Kim Jong-il/Kim jong-un).

• Just as a general resource, the DPRK makes most of their own publications (magazines/books/photo albums/etc.) available through this website. They have a series called "Understanding Korea", the third of which deals with their political system from their point of view. Again, you might have trouble accessing this if you're in the States or some other countries.