Why use FIFO to read analog signal?

Why must we use FIFO to obtain analog signal from FPGA? I can use a normal method of calling the FPGA using Read/Write Control and the results in a waveform chart is much more smoother. 

I've uploaded 2 real-time VIs with and without FIFO. MQ convert is the subVI in my computer. Do have a look at it...
My waveform chart is done that way so that I can easily export to excel ( i think) but this is a different problem in a different topic http://forums.ni.com/t5/LabVIEW/3-Waveform-Chart-to-a-Single-Excel-but-in-Different-Spreadsheet/m-p/...
Attachments:
Real-Time With FIFO.vi ‏397 KB
Real-Time without FIFO.vi ‏391 KB
MQ Convert.vi ‏19 KB

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