Performance generally indicates acceptance of an offer
Alice sells cookies for 1 unit of currency per item. Bob comes by and gives her 5 and says "5 cookies." By accepting the cash, Alice is now bound to deliver 5 cookies to Bob. Simple, easy, done? No, in contract terms: Alice was offering a contract, Bob accepted it by performing the indicated payment.
Charly sells computers with terms of service taped to the side. Dorothy comes by, sees the contract on the side, and buys it anyway. The Terms of Service now apply, since they were quite visible.
Echo-Corp is offering tools on the internet. The form for ordering reads the items ordered, then a single line, well indicated by being written in bold and a little larger than normal font, "by ordering below, I accept the ToS", and then the "Buy" button. Felicia orders one via the form, and is bound by the ToS.
Order of indications can matter, especially where things are not obvious
Gang Inc. is offering cookies, but the order of buttons is different: the "Order now" is above the "ToS apply". Herbert orders, and as a result of the button being above the order button, the ToS don't apply in germany - ToS need to be advertised in a well visible script, and before the order/buy button. This was discussed in the Urteil des I. Zivilsenats vom 14.6.2006 - I ZR 75/03 - which also did establish that you don't need a checkbox, only advertize the ToS properly and in sufficient script, even if you link to them.
Also, the caption of "order now" is by itself not sufficient in Germany - it should read something akin to "order (payment required)" [Bestellen (Zahlungspflichtig)] or "buy now" [jetzt kaufen], indicating that payment is mandatory under BGB § 312j. This stems from a German language quirk: Order [bestellen] is technically a request to have something, and constitutes an offer to the other party while buy [kaufen] is the act of accepting and paying for something.
Checkbox can make defense easier, if used properly
According to German law, the checkbox isn't needed, but it helps the seller - if Gang Inc. puts the checkbox there and does not process till it is ticked (and the text is sufficient to be the "well readable" link to the ToS), then this makes it much easier for the company to point to the ToS (assuming they fix their order button to be compliant).
This square can't be hooked by default, so placing the hook is an act by the user - and by this action verifying that the user does accept the ToS or an ad cookie or whatever. Such was found in the series of Planet 49 GmbH cases - which started in 2013 over ad cookies and ended in 2019 in front of the highest EU Court, verifying the previous BGH result, and thus leading to Urteil des I. Zivilsenats vom 28.5.2020 - I ZR 7/16.