As you can understand, there is a lot of debate about the 'earthly' reasons for various Old Testament commandments. It used to be in vogue to try and find various health reasons behind the food code, for example.

For my money, the best approach is one that is typified by the structuralist analysis done by Mary Douglas. This sees Leviticus as part of a system into which things (objects/deeds) are categorised into clean/unclean/holy.

In this kind of view, some of what is going on in Leviticus is not "this is absolutely wrong", but "this action/object has this kind of value because its been assigned that value in the system". In some ways this parallels philosophical theories of language going back to Saussure.

Anyway, your first specific example would then function as part of a system in which things are not to be 'mixed', part of a larger system in which clean and unclean are not to associate, and primarily that God associates with things/people that are holy.

Your second verse is much more straightfoward, in my opinion, as it is part of the economic provisions of Leviticus. There are two verses that have similar instructions, 19:9 and 23:22. The first is explained further in 19:10, "you shall leave them for the poor and for the resident-alien: I am the Lord your God". It's a form of social welfare that leaves some of the produce for the poor to gather. The second occurence of a similar verse in 23:22 has a similar reminder that its purpose is social justice, and occurs in the context of a festival to give offerings from the harvest.