Why are these semi quavers written like this?
It is part of a string quartet
There are, at least in a way, two answers to this question. This first is that it simply saves some ink and some performer eyestrain because writing it out in full would involve double beams and each note written twice.
The second answer gets at the psychological or interpretive meaning of a notation like this, and is, as such, more subjective. I know that for me, and I think for a lot of other string players, there is a slight difference in stylistic interpretation between notation like that in your question and the expanded version it functions as an abbreviation of. When I play a passage like this, I generally try to have slightly more of a sense that I'm playing a line that is essentially eighth notes, but that each eighth note is just doubled for added volume and intensity. I slightly accent the first of each doubled note and try to think of the line in its more basic, purely eighth note form. Obviously, this is a highly nuanced idea, and may not be especially obvious to a listener, I'm not sure. All I know is that I tend to approach passages like this as an eighth note idea that is simply being doubled, as opposed to a sixteenth note idea. Does that make any sense? At least subjectively, it feels like a relatively strong difference of emphasis.
It means to play each note twice, as 16ths. Sort of a Stage 1 tremolo I suppose! But very specifically just two 16ths for each 8th. Very common notation from the Baroque era onwards. Used for all orchestral instruments.
They look like measured tremolos on a single note. Here's some information:
How to interpret half notes combined with thirty-second stems? [This one is mostly about unmeasured tremolos]
Note that those examples mostly show tremolos between two notes, but it's perfectly normal to perform one on the same note. The specific technique that you will need depends on the instrument that you play.
I seem to have attracted a downvote, so perhaps I should be clearer. Tremolo doesn't mean "play as many notes as you can". The number of slashes through the note stem actually means something. The term I've heard used is "measured tremolo", as opposed to an "unmeasured tremolo".
Single slashes are measured tremolos. If they are quavers, play semiquavers. If they are crotchets, play quavers. It gets a little more vague with minims and semibreves, but I'd generally play quavers. If they wanted crotchets or minims, they should just write them out.
Double slashes are also measured tremolos, but at twice the speed.
Triple slashes are your classic "play all the notes" unmeasured tremolos. The exception would be if the tempo is really slow, and the symbol is on semibreves, which can be a little ambiguous.
I'm not claiming this is an authoritative source, but it seems close to what I've heard: https://easymusictheory.wordpress.com/tag/measured-tremolo/.
Also, for what it's worth, Sibelius interprets tremolos in a similar fashion.