source code:

MODULE_ALIAS("platform:sm501");

static const struct of_device_id of_sm501_match_tbl[] = {
    { .compatible = "smi,sm501", },
    { /* end */ }
};
MODULE_DEVICE_TABLE(of, of_sm501_match_tbl);

location: http://lxr.free-electrons.com/source/drivers/mfd/sm501.c#L1711

Since there is the MODULE_DEVICE_TABLE(), why  the author additionally

adds the MODULE_ALIAS()?

What exactly is this MODULE_ALIAS() used for?

Short answer: to re-enable auto loading of the module, Please refer subject
line here:
https://lab.nexedi.cn/kirr/linux/commit/4f46d6e7e5ffbce0ee1d1a80767fdf45e56cc863

Long answer:
https://kerneltweaks.wordpress.com/2014/03/30/platform-device-and-platform-driver-linux/

References:
Review the section kmod vs kerneld: http://lpic2.unix.nl/ch02s03.html

What is MODULE_ALIAS:
http://stackoverflow.com/questions/22778879/what-is-module-alias-in-linux-device-driver-code

Review Alias section:
https://wiki.debian.org/Modules#Automatic_loading_of_modules

Review Identifying Device Driver Modules:
http://free-electrons.com/doc/udev.pdf

Useful to know:
1 - https://frankpzh.wordpress.com/2011/04/16/kmod-udev-and-modprobe/
2 -
https://www.quora.com/What-is-the-difference-between-platform-device-drivers-and-normal-device-drivers
3 - http://www.atmel.com/Images/doc32098.pdf

Where it all began:
http://www.linuxjournal.com/node/5604/print

Hope this helps.

Thanks - Aruna
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