Martin, 33, has pitched to a 3.08 ERA and 1.03 WHIP with 43 strikeouts and just four walks in 38 innings this season. He has four saves and 12 holds with just one blown save, so while he's not really known as a high-leverage guy, it's possible the Braves plan on using him in the late innings, perhaps maybe even as a closer.
The Braves have a decent lead in the NL East, but they have had issues with blown saves. Current closer Luke Jackson has 17 saves and seven blown saves and that's too high for comfort. It's possible that the Braves are intending to add multiple arms and Martin is their depth acquisition while they go out and get a so-called "proven closer" to take over in the ninth. Then Jackson and Martin would be part of the bridge to said closer.
Allard, 21, was roughed up in a big way at the major-league level this season with a 12.38 ERA and 2.88 WHIP in eight innings, but he's very young, it's a small sample and there's upside. At one point Baseball Prospectus had him as a top-25 prospect in baseball. In 20 starts for Triple-A Gwinnett this season, he's got a 4.17 ERA, 1.41 WHIP and 98 strikeouts against 36 walks in 110 innings. The Rangers will be looking to get him back into 2018 form, when he had a 2.72 ERA in Triple-A.