Open a .vcf address book
A .vcf is the file you get when you export contacts from a phone, from Gmail or from iCloud. Double-click it and the computer tries to import them into your own address book, which is almost never what you wanted. Here you just look at them: all together, searchable, and if you need them in Excel you download them as CSV. The file stays on your device.
Looking is not importing
That is the difference no program lets you have. If a colleague sends you a client's contact list and you double-click it, those contacts go into your address book and from there onto all your devices: taking them out again one by one is a job. Here the file is only read and shown: your own address book is not touched, and when you close the page nothing is left.
Names with accents that arrive in pieces
If the file comes from an Android phone it is nearly always in the old 2.1 format, where accented characters are written in a way of their own: «Nicol=C3=B2» means «Nicolò». Plenty of readers do not decode it and show you an address book in pieces. Here it is decoded, and the alphabet the file declares is listened to as well: on older phones the same code means another letter, and reading it the wrong way does not give you a wrong accent, it gives you a broken character.
Duplicate numbers, contacts with no name, photos
An exported address book is nearly always a bit untidy. Here you are told how many contacts share the same number (the comparison is on the last nine digits, so «+39 333 1234567» and «3331234567» are the same person), contacts that have only a number and no name are shown anyway instead of disappearing, and the photos inside the file are visible: they are written in there, they do not come from the internet.
Taking it into Excel, without losing the leading zeros
The button downloads a CSV with one column per field, already using the separator your language's Excel expects (in Italy the semicolon: with a comma everything would end up in a single column). Watch out for one thing: if you open the CSV with a double click, Excel eats the leading zero of landline numbers and turns long numbers into scientific notation. To avoid that go through CSV to Excel, which keeps numbers as text.
What it does not do
It does not change the file and does not make a new one: it does not merge duplicates, does not fix numbers, does not import anything anywhere. It does not read your phone's address book: it only reads the file you hand it. If the file you have is an .eml or an .msg, or an Excel sheet, there are pages made for those.